-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. There are
separate text files for the pcregrep and pcretest commands.
-----------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions


INTRODUCTION

       The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a  few  differences.  The current implementation of PCRE (release
       6.x) corresponds approximately with Perl  5.8,  including  support  for
       UTF-8 encoded strings and Unicode general category properties. However,
       this support has to be explicitly enabled; it is not the default.

       In addition to the Perl-compatible matching function,  PCRE  also  con-
       tains  an  alternative matching function that matches the same compiled
       patterns in a different way. In certain circumstances, the  alternative
       function  has  some  advantages.  For  a discussion of the two matching
       algorithms, see the pcrematching page.

       PCRE is written in C and released as a C library. A  number  of  people
       have  written  wrappers and interfaces of various kinds. In particular,
       Google Inc.  have provided a comprehensive C++  wrapper.  This  is  now
       included as part of the PCRE distribution. The pcrecpp page has details
       of this interface. Other people's contributions can  be  found  in  the
       Contrib directory at the primary FTP site, which is:

       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

       Details  of  exactly which Perl regular expression features are and are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern and pcrecompat pages.

       Some  features  of  PCRE can be included, excluded, or changed when the
       library is built. The pcre_config() function makes it  possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcrebuild page. Documentation about  build-
       ing  PCRE for various operating systems can be found in the README file
       in the source distribution.

       The library contains a number of undocumented  internal  functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre_", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external  symbols  are  exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.


USER DOCUMENTATION

       The user documentation for PCRE comprises a number  of  different  sec-
       tions.  In the "man" format, each of these is a separate "man page". In
       the HTML format, each is a separate page, linked from the  index  page.
       In  the  plain text format, all the sections are concatenated, for ease
       of searching. The sections are as follows:

         pcre              this document
         pcreapi           details of PCRE's native C API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper
         pcregrep          description of the pcregrep command
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the sample program
         pcrestack         discussion of stack usage
         pcretest          description of the pcretest testing command

       In  addition,  in the "man" and HTML formats, there is a short page for
       each C library function, listing its arguments and results.


LIMITATIONS

       There are some size limitations in PCRE but it is hoped that they  will
       never in practice be relevant.

       The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
       is compiled with the default internal linkage size of 2. If you want to
       process  regular  expressions  that are truly enormous, you can compile
       PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
       the  source  distribution and the pcrebuild documentation for details).
       In these cases the limit is substantially larger.  However,  the  speed
       of execution will be slower.

       All  values in repeating quantifiers must be less than 65536. The maxi-
       mum compiled length of subpattern with  an  explicit  repeat  count  is
       30000 bytes. The maximum number of capturing subpatterns is 65535.

       There  is  no limit to the number of non-capturing subpatterns, but the
       maximum depth of nesting of  all  kinds  of  parenthesized  subpattern,
       including capturing subpatterns, assertions, and other types of subpat-
       tern, is 200.

       The maximum length of name for a named subpattern is 32, and the  maxi-
       mum number of named subpatterns is 10000.

       The  maximum  length of a subject string is the largest positive number
       that an integer variable can hold. However, when using the  traditional
       matching function, PCRE uses recursion to handle subpatterns and indef-
       inite repetition.  This means that the available stack space may  limit
       the size of a subject string that can be processed by certain patterns.
       For a discussion of stack issues, see the pcrestack documentation.


UTF-8 AND UNICODE PROPERTY SUPPORT

       From release 3.3, PCRE has  had  some  support  for  character  strings
       encoded  in the UTF-8 format. For release 4.0 this was greatly extended
       to cover most common requirements, and in release 5.0  additional  sup-
       port for Unicode general category properties was added.

       In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
       support in the code, and, in addition,  you  must  call  pcre_compile()
       with  the PCRE_UTF8 option flag. When you do this, both the pattern and
       any subject strings that are matched against it are  treated  as  UTF-8
       strings instead of just strings of bytes.

       If  you compile PCRE with UTF-8 support, but do not use it at run time,
       the library will be a bit bigger, but the additional run time  overhead
       is  limited  to testing the PCRE_UTF8 flag in several places, so should
       not be very large.

       If PCRE is built with Unicode character property support (which implies
       UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
       ported.  The available properties that can be tested are limited to the
       general  category  properties such as Lu for an upper case letter or Nd
       for a decimal number, the Unicode script names such as Arabic  or  Han,
       and  the  derived  properties  Any  and L&. A full list is given in the
       pcrepattern documentation. Only the short names for properties are sup-
       ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
       ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may
       optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
       does not support this.

       The following comments apply when PCRE is running in UTF-8 mode:

       1. When you set the PCRE_UTF8 flag, the strings passed as patterns  and
       subjects  are  checked for validity on entry to the relevant functions.
       If an invalid UTF-8 string is passed, an error return is given. In some
       situations,  you  may  already  know  that  your strings are valid, and
       therefore want to skip these checks in order to improve performance. If
       you  set  the  PCRE_NO_UTF8_CHECK  flag at compile time or at run time,
       PCRE assumes that the pattern or subject  it  is  given  (respectively)
       contains  only valid UTF-8 codes. In this case, it does not diagnose an
       invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE  when
       PCRE_NO_UTF8_CHECK  is set, the results are undefined. Your program may
       crash.

       2. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
       two-byte UTF-8 character if the value is greater than 127.

       3.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
       characters for values greater than \177.

       4. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
       vidual bytes, for example: \x{100}{3}.

       5.  The dot metacharacter matches one UTF-8 character instead of a sin-
       gle byte.

       6. The escape sequence \C can be used to match a single byte  in  UTF-8
       mode,  but  its  use can lead to some strange effects. This facility is
       not available in the alternative matching function, pcre_dfa_exec().

       7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
       test  characters of any code value, but the characters that PCRE recog-
       nizes as digits, spaces, or word characters  remain  the  same  set  as
       before, all with values less than 256. This remains true even when PCRE
       includes Unicode property support, because to do otherwise  would  slow
       down  PCRE in many common cases. If you really want to test for a wider
       sense of, say, "digit", you must use Unicode  property  tests  such  as
       \p{Nd}.

       8.  Similarly,  characters that match the POSIX named character classes
       are all low-valued characters.

       9. Case-insensitive matching applies only to  characters  whose  values
       are  less than 128, unless PCRE is built with Unicode property support.
       Even when Unicode property support is available, PCRE  still  uses  its
       own  character  tables when checking the case of low-valued characters,
       so as not to degrade performance.  The Unicode property information  is
       used only for characters with higher values. Even when Unicode property
       support is available, PCRE supports case-insensitive matching only when
       there  is  a  one-to-one  mapping between a letter's cases. There are a
       small number of many-to-one mappings in Unicode;  these  are  not  sup-
       ported by PCRE.


AUTHOR

       Philip Hazel
       University Computing Service,
       Cambridge CB2 3QG, England.

       Putting  an actual email address here seems to have been a spam magnet,
       so I've taken it away. If you want to email me, use my initial and sur-
       name, separated by a dot, at the domain ucs.cam.ac.uk.

Last updated: 05 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREBUILD(3)                                                      PCREBUILD(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE BUILD-TIME OPTIONS

       This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. They are all selected, or  dese-
       lected, by providing options to the configure script that is run before
       the make command. The complete list of  options  for  configure  (which
       includes  the  standard  ones such as the selection of the installation
       directory) can be obtained by running

         ./configure --help

       The following sections describe certain options whose names begin  with
       --enable  or  --disable. These settings specify changes to the defaults
       for the configure command. Because of the  way  that  configure  works,
       --enable  and  --disable  always  come  in  pairs, so the complementary
       option always exists as well, but as it specifies the  default,  it  is
       not described.


C++ SUPPORT

       By default, the configure script will search for a C++ compiler and C++
       header files. If it finds them, it automatically builds the C++ wrapper
       library for PCRE. You can disable this by adding

         --disable-cpp

       to the configure command.


UTF-8 SUPPORT

       To build PCRE with support for UTF-8 character strings, add

         --enable-utf8

       to  the  configure  command.  Of  itself, this does not make PCRE treat
       strings as UTF-8. As well as compiling PCRE with this option, you  also
       have  have to set the PCRE_UTF8 option when you call the pcre_compile()
       function.


UNICODE CHARACTER PROPERTY SUPPORT

       UTF-8 support allows PCRE to process character values greater than  255
       in  the  strings that it handles. On its own, however, it does not pro-
       vide any facilities for accessing the properties of such characters. If
       you  want  to  be able to use the pattern escapes \P, \p, and \X, which
       refer to Unicode character properties, you must add

         --enable-unicode-properties

       to the configure command. This implies UTF-8 support, even if you  have
       not explicitly requested it.

       Including  Unicode  property  support  adds around 90K of tables to the
       PCRE library, approximately doubling its size. Only the  general  cate-
       gory  properties  such as Lu and Nd are supported. Details are given in
       the pcrepattern documentation.


CODE VALUE OF NEWLINE

       By default, PCRE interprets character 10 (linefeed, LF)  as  indicating
       the  end  of  a line. This is the normal newline character on Unix-like
       systems. You can compile PCRE to use character 13 (carriage return, CR)
       instead, by adding

         --enable-newline-is-cr

       to  the  configure  command.  There  is  also  a --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add

         --enable-newline-is-crlf

       to  the  configure command. Whatever line ending convention is selected
       when PCRE is built can be overridden when  the  library  functions  are
       called.  At  build time it is conventional to use the standard for your
       operating system.


BUILDING SHARED AND STATIC LIBRARIES

       The PCRE building process uses libtool to build both shared and  static
       Unix  libraries by default. You can suppress one of these by adding one
       of

         --disable-shared
         --disable-static

       to the configure command, as required.


POSIX MALLOC USAGE

       When PCRE is called through the POSIX interface (see the pcreposix doc-
       umentation),  additional  working  storage  is required for holding the
       pointers to capturing substrings, because PCRE requires three  integers
       per  substring,  whereas  the POSIX interface provides only two. If the
       number of expected substrings is small, the wrapper function uses space
       on the stack, because this is faster than using malloc() for each call.
       The default threshold above which the stack is no longer used is 10; it
       can be changed by adding a setting such as

         --with-posix-malloc-threshold=20

       to the configure command.


HANDLING VERY LARGE PATTERNS

       Within  a  compiled  pattern,  offset values are used to point from one
       part to another (for example, from an opening parenthesis to an  alter-
       nation  metacharacter).  By default, two-byte values are used for these
       offsets, leading to a maximum size for a  compiled  pattern  of  around
       64K.  This  is sufficient to handle all but the most gigantic patterns.
       Nevertheless, some people do want to process enormous patterns,  so  it
       is  possible  to compile PCRE to use three-byte or four-byte offsets by
       adding a setting such as

         --with-link-size=3

       to the configure command. The value given must be 2,  3,  or  4.  Using
       longer  offsets slows down the operation of PCRE because it has to load
       additional bytes when handling them.

       If you build PCRE with an increased link size, test 2 (and  test  5  if
       you  are using UTF-8) will fail. Part of the output of these tests is a
       representation of the compiled pattern, and this changes with the  link
       size.


AVOIDING EXCESSIVE STACK USAGE

       When matching with the pcre_exec() function, PCRE implements backtrack-
       ing by making recursive calls to an internal function  called  match().
       In  environments  where  the size of the stack is limited, this can se-
       verely limit PCRE's operation. (The Unix environment does  not  usually
       suffer from this problem, but it may sometimes be necessary to increase
       the maximum stack size.  There is a discussion in the  pcrestack  docu-
       mentation.)  An alternative approach to recursion that uses memory from
       the heap to remember data, instead of using recursive  function  calls,
       has  been  implemented to work round the problem of limited stack size.
       If you want to build a version of PCRE that works this way, add

         --disable-stack-for-recursion

       to the configure command. With this configuration, PCRE  will  use  the
       pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
       ment functions. Separate functions are provided because  the  usage  is
       very  predictable:  the  block sizes requested are always the same, and
       the blocks are always freed in reverse order. A calling  program  might
       be  able  to implement optimized functions that perform better than the
       standard malloc() and  free()  functions.  PCRE  runs  noticeably  more
       slowly when built in this way. This option affects only the pcre_exec()
       function; it is not relevant for the the pcre_dfa_exec() function.


LIMITING PCRE RESOURCE USAGE

       Internally, PCRE has a function called match(), which it calls  repeat-
       edly   (sometimes   recursively)  when  matching  a  pattern  with  the
       pcre_exec() function. By controlling the maximum number of  times  this
       function  may be called during a single matching operation, a limit can
       be placed on the resources used by a single call  to  pcre_exec().  The
       limit  can be changed at run time, as described in the pcreapi documen-
       tation. The default is 10 million, but this can be changed by adding  a
       setting such as

         --with-match-limit=500000

       to   the   configure  command.  This  setting  has  no  effect  on  the
       pcre_dfa_exec() matching function.

       In some environments it is desirable to limit the  depth  of  recursive
       calls of match() more strictly than the total number of calls, in order
       to restrict the maximum amount of stack (or heap,  if  --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it defaults to the value that  is  set  for  --with-match-limit,  which
       imposes  no  additional constraints. However, you can set a lower limit
       by adding, for example,

         --with-match-limit-recursion=10000

       to the configure command. This value can  also  be  overridden  at  run
       time.


USING EBCDIC CODE

       PCRE  assumes  by  default that it will run in an environment where the
       character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
       PCRE  can,  however,  be  compiled  to  run in an EBCDIC environment by
       adding

         --enable-ebcdic

       to the configure command.

Last updated: 06 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREMATCHING(3)                                                PCREMATCHING(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE for matching a compiled regular expression against a given sub-
       ject  string.  The  "standard"  algorithm  is  the  one provided by the
       pcre_exec() function.  This works in the same was  as  Perl's  matching
       function, and provides a Perl-compatible matching operation.

       An  alternative  algorithm is provided by the pcre_dfa_exec() function;
       this operates in a different way, and is not  Perl-compatible.  It  has
       advantages  and disadvantages compared with the standard algorithm, and
       these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the DFA algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book Mastering  Regular  Expres-
       sions,  the  standard  algorithm  is  an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.


THE DFA MATCHING ALGORITHM

       DFA stands for "deterministic finite automaton", but you do not need to
       understand the origins of that name. This algorithm conducts a breadth-
       first search of the tree. Starting from the first matching point in the
       subject,  it scans the subject string from left to right, once, charac-
       ter by character, and as it does  this,  it  remembers  all  the  paths
       through the tree that represent valid matches.

       The  scan  continues until either the end of the subject is reached, or
       there are no more unterminated paths. At this point,  terminated  paths
       represent  the different matching possibilities (if there are none, the
       match has failed).  Thus, if there is more  than  one  possible  match,
       this algorithm finds all of them, and in particular, it finds the long-
       est. In PCRE, there is an option to stop the algorithm after the  first
       match (which is necessarily the shortest) has been found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)

       is matched against the string "the caterpillar catchment",  the  result
       will  be the three strings "cat", "cater", and "caterpillar" that start
       at the fourth character of the subject. The algorithm does not automat-
       ically move on to find matches that start at later positions.

       There are a number of features of PCRE regular expressions that are not
       supported by the DFA matching algorithm. They are as follows:

       1. Because the algorithm finds all  possible  matches,  the  greedy  or
       ungreedy  nature  of repetition quantifiers is not relevant. Greedy and
       ungreedy quantifiers are treated in exactly the same way.

       2. When dealing with multiple paths through the tree simultaneously, it
       is  not  straightforward  to  keep track of captured substrings for the
       different matching possibilities, and  PCRE's  implementation  of  this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, back references within the  pat-
       tern are not supported, and cause errors if encountered.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition are not supported.

       5. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always -1.

       6.  The \C escape sequence, which (in the standard algorithm) matches a
       single byte, even in UTF-8 mode, is not supported because the DFA algo-
       rithm moves through the subject string one character at a time, for all
       active paths through the tree.


ADVANTAGES OF THE DFA ALGORITHM

       Using the DFA matching algorithm provides the following advantages:

       1. All possible matches (at a single point in the subject) are automat-
       ically  found,  and  in particular, the longest match is found. To find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2.  There is much better support for partial matching. The restrictions
       on the content of the pattern that apply when using the standard  algo-
       rithm  for partial matching do not apply to the DFA algorithm. For non-
       anchored patterns, the starting position of a partial match  is  avail-
       able.

       3.  Because  the  DFA algorithm scans the subject string just once, and
       never needs to backtrack, it is possible  to  pass  very  long  subject
       strings  to  the matching function in several pieces, checking for par-
       tial matching each time.


DISADVANTAGES OF THE DFA ALGORITHM

       The DFA algorithm suffers from a number of disadvantages:

       1. It is substantially slower than  the  standard  algorithm.  This  is
       partly  because  it has to search for all possible matches, but is also
       because it is less susceptible to optimization.

       2. Capturing parentheses and back references are not supported.

       3. The "atomic group" feature of PCRE regular expressions is supported,
       but  does not provide the advantage that it does for the standard algo-
       rithm.

Last updated: 06 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREAPI(3)                                                          PCREAPI(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE NATIVE API

       #include <pcre.h>

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre_extra *pcre_study(const pcre *code, int options,
            const char **errptr);

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       void pcre_free_substring(const char *stringptr);

       void pcre_free_substring_list(const char **stringptr);

       const unsigned char *pcre_maketables(void);

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

       int pcre_refcount(pcre *code, int adjust);

       int pcre_config(int what, void *where);

       char *pcre_version(void);

       void *(*pcre_malloc)(size_t);

       void (*pcre_free)(void *);

       void *(*pcre_stack_malloc)(size_t);

       void (*pcre_stack_free)(void *);

       int (*pcre_callout)(pcre_callout_block *);


PCRE API OVERVIEW

       PCRE has its own native API, which is described in this document. There
       is also a set of wrapper functions that correspond to the POSIX regular
       expression  API.  These  are  described in the pcreposix documentation.
       Both of these APIs define a set of C function calls. A C++  wrapper  is
       distributed with PCRE. It is documented in the pcrecpp page.

       The  native  API  C  function prototypes are defined in the header file
       pcre.h, and on Unix systems the library itself is called  libpcre.   It
       can normally be accessed by adding -lpcre to the command for linking an
       application  that  uses  PCRE.  The  header  file  defines  the  macros
       PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
       bers for the library.  Applications can use these  to  include  support
       for different releases of PCRE.

       The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
       pcre_exec() are used for compiling and matching regular expressions  in
       a  Perl-compatible  manner. A sample program that demonstrates the sim-
       plest way of using them is provided in the file  called  pcredemo.c  in
       the  source distribution. The pcresample documentation describes how to
       run it.

       A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble,  is  also provided. This uses a different algorithm for the match-
       ing. The alternative algorithm finds all possible matches (at  a  given
       point in the subject). However, this algorithm does not return captured
       substrings. A description of the  two  matching  algorithms  and  their
       advantages  and  disadvantages  is given in the pcrematching documenta-
       tion.

       In addition to the main compiling and  matching  functions,  there  are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

         pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

       pcre_free_substring() and pcre_free_substring_list() are also provided,
       to free the memory used for extracted strings.

       The  function  pcre_maketables()  is  used  to build a set of character
       tables  in  the  current  locale   for   passing   to   pcre_compile(),
       pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
       provided for specialist use.  Most  commonly,  no  special  tables  are
       passed,  in  which case internal tables that are generated when PCRE is
       built are used.

       The function pcre_fullinfo() is used to find out  information  about  a
       compiled  pattern; pcre_info() is an obsolete version that returns only
       some of the available information, but is retained for  backwards  com-
       patibility.   The function pcre_version() returns a pointer to a string
       containing the version of PCRE and its date of release.

       The function pcre_refcount() maintains a  reference  count  in  a  data
       block  containing  a compiled pattern. This is provided for the benefit
       of object-oriented applications.

       The global variables pcre_malloc and pcre_free  initially  contain  the
       entry  points  of  the  standard malloc() and free() functions, respec-
       tively. PCRE calls the memory management functions via these variables,
       so  a  calling  program  can replace them if it wishes to intercept the
       calls. This should be done before calling any PCRE functions.

       The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
       indirections  to  memory  management functions. These special functions
       are used only when PCRE is compiled to use  the  heap  for  remembering
       data, instead of recursive function calls, when running the pcre_exec()
       function. See the pcrebuild documentation for  details  of  how  to  do
       this.  It  is  a non-standard way of building PCRE, for use in environ-
       ments that have limited stacks. Because of the greater  use  of  memory
       management,  it  runs  more  slowly. Separate functions are provided so
       that special-purpose external code can be  used  for  this  case.  When
       used,  these  functions  are always called in a stack-like manner (last
       obtained, first freed), and always for memory blocks of the same  size.
       There  is  a discussion about PCRE's stack usage in the pcrestack docu-
       mentation.

       The global variable pcre_callout initially contains NULL. It can be set
       by  the  caller  to  a "callout" function, which PCRE will then call at
       specified points during a matching operation. Details are given in  the
       pcrecallout documentation.


NEWLINES
       PCRE supports three different conventions for indicating line breaks in
       strings: a single CR character, a single LF character, or the two-char-
       acter  sequence  CRLF.  All  three  are used as "standard" by different
       operating systems.  When PCRE is built, a default can be specified. The
       default  default  is  LF, which is the Unix standard. When PCRE is run,
       the default can be overridden, either when a pattern  is  compiled,  or
       when it is matched.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter or pair of characters that indicate a line break".


MULTITHREADING

       The PCRE functions can be used in  multi-threading  applications,  with
       the  proviso  that  the  memory  management  functions  pointed  to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, are shared by all threads.

       The compiled form of a regular expression is not altered during  match-
       ing, so the same compiled pattern can safely be used by several threads
       at once.


SAVING PRECOMPILED PATTERNS FOR LATER USE

       The compiled form of a regular expression can be saved and re-used at a
       later  time,  possibly by a different program, and even on a host other
       than the one on which  it  was  compiled.  Details  are  given  in  the
       pcreprecompile documentation.


CHECKING BUILD-TIME OPTIONS

       int pcre_config(int what, void *where);

       The  function pcre_config() makes it possible for a PCRE client to dis-
       cover which optional features have been compiled into the PCRE library.
       The  pcrebuild documentation has more details about these optional fea-
       tures.

       The first argument for pcre_config() is an  integer,  specifying  which
       information is required; the second argument is a pointer to a variable
       into which the information is  placed.  The  following  information  is
       available:

         PCRE_CONFIG_UTF8

       The  output is an integer that is set to one if UTF-8 support is avail-
       able; otherwise it is set to zero.

         PCRE_CONFIG_UNICODE_PROPERTIES

       The output is an integer that is set to  one  if  support  for  Unicode
       character properties is available; otherwise it is set to zero.

         PCRE_CONFIG_NEWLINE

       The  output  is  an integer whose value specifies the default character
       sequence that is recognized as meaning "newline". The three values that
       are supported are: 10 for LF, 13 for CR, and 3338 for CRLF. The default
       should normally be the standard sequence for your operating system.

         PCRE_CONFIG_LINK_SIZE

       The output is an integer that contains the number  of  bytes  used  for
       internal linkage in compiled regular expressions. The value is 2, 3, or
       4. Larger values allow larger regular expressions to  be  compiled,  at
       the  expense  of  slower matching. The default value of 2 is sufficient
       for all but the most massive patterns, since  it  allows  the  compiled
       pattern to be up to 64K in size.

         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

       The  output  is  an integer that contains the threshold above which the
       POSIX interface uses malloc() for output vectors. Further  details  are
       given in the pcreposix documentation.

         PCRE_CONFIG_MATCH_LIMIT

       The output is an integer that gives the default limit for the number of
       internal matching function calls in a  pcre_exec()  execution.  Further
       details are given with pcre_exec() below.

         PCRE_CONFIG_MATCH_LIMIT_RECURSION

       The  output is an integer that gives the default limit for the depth of
       recursion when calling the internal matching function in a  pcre_exec()
       execution. Further details are given with pcre_exec() below.

         PCRE_CONFIG_STACKRECURSE

       The  output is an integer that is set to one if internal recursion when
       running pcre_exec() is implemented by recursive function calls that use
       the  stack  to remember their state. This is the usual way that PCRE is
       compiled. The output is zero if PCRE was compiled to use blocks of data
       on  the  heap  instead  of  recursive  function  calls.  In  this case,
       pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
       blocks on the heap, thus avoiding the use of the stack.


COMPILING A PATTERN

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the  two interfaces is that pcre_compile2() has an additional argument,
       errorcodeptr, via which a numerical error code can be returned.

       The pattern is a C string terminated by a binary zero, and is passed in
       the  pattern  argument.  A  pointer to a single block of memory that is
       obtained via pcre_malloc is returned. This contains the  compiled  code
       and related data. The pcre type is defined for the returned block; this
       is a typedef for a structure whose contents are not externally defined.
       It is up to the caller to free the memory (via pcre_free) when it is no
       longer required.

       Although the compiled code of a PCRE regex is relocatable, that is,  it
       does not depend on memory location, the complete pcre data block is not
       fully relocatable, because it may contain a copy of the tableptr  argu-
       ment, which is an address (see below).

       The options argument contains independent bits that affect the compila-
       tion. It should be zero if  no  options  are  required.  The  available
       options  are  described  below. Some of them, in particular, those that
       are compatible with Perl, can also be set and  unset  from  within  the
       pattern  (see  the  detailed  description in the pcrepattern documenta-
       tion). For these options, the contents of the options  argument  speci-
       fies  their initial settings at the start of compilation and execution.
       The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at  the  time
       of matching as well as at compile time.

       If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
       sets the variable pointed to by errptr to point to a textual error mes-
       sage. This is a static string that is part of the library. You must not
       try to free it. The offset from the start of the pattern to the charac-
       ter where the error was discovered is placed in the variable pointed to
       by  erroffset,  which must not be NULL. If it is, an immediate error is
       given.

       If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
       codeptr  argument is not NULL, a non-zero error code number is returned
       via this argument in the event of an error. This is in addition to  the
       textual error message. Error codes and messages are listed below.

       If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
       character tables that are  built  when  PCRE  is  compiled,  using  the
       default  C  locale.  Otherwise, tableptr must be an address that is the
       result of a call to pcre_maketables(). This value is  stored  with  the
       compiled  pattern,  and used again by pcre_exec(), unless another table
       pointer is passed to it. For more discussion, see the section on locale
       support below.

       This  code  fragment  shows a typical straightforward call to pcre_com-
       pile():

         pcre *re;
         const char *error;
         int erroffset;
         re = pcre_compile(
           "^A.*Z",          /* the pattern */
           0,                /* default options */
           &error,           /* for error message */
           &erroffset,       /* for error offset */
           NULL);            /* use default character tables */

       The following names for option bits are defined in  the  pcre.h  header
       file:

         PCRE_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is constrained to match only at the first matching point in the  string
       that  is being searched (the "subject string"). This effect can also be
       achieved by appropriate constructs in the pattern itself, which is  the
       only way to do it in Perl.

         PCRE_AUTO_CALLOUT

       If this bit is set, pcre_compile() automatically inserts callout items,
       all with number 255, before each pattern item. For  discussion  of  the
       callout facility, see the pcrecallout documentation.

         PCRE_CASELESS

       If  this  bit is set, letters in the pattern match both upper and lower
       case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
       changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
       always understands the concept of case for characters whose values  are
       less  than 128, so caseless matching is always possible. For characters
       with higher values, the concept of case is supported if  PCRE  is  com-
       piled  with Unicode property support, but not otherwise. If you want to
       use caseless matching for characters 128 and  above,  you  must  ensure
       that  PCRE  is  compiled  with Unicode property support as well as with
       UTF-8 support.

         PCRE_DOLLAR_ENDONLY

       If this bit is set, a dollar metacharacter in the pattern matches  only
       at  the  end  of the subject string. Without this option, a dollar also
       matches immediately before a newline at the end of the string (but  not
       before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
       if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
       Perl, and no way to set it within a pattern.

         PCRE_DOTALL

       If this bit is set, a dot metacharater in the pattern matches all char-
       acters, including those that indicate newline. Without it, a  dot  does
       not  match  when  the  current position is at a newline. This option is
       equivalent to Perl's /s option, and it can be changed within a  pattern
       by  a (?s) option setting. A negative class such as [^a] always matches
       newlines, independent of the setting of this option.

         PCRE_DUPNAMES

       If this bit is set, names used to identify capturing  subpatterns  need
       not be unique. This can be helpful for certain types of pattern when it
       is known that only one instance of the named  subpattern  can  ever  be
       matched.  There  are  more details of named subpatterns below; see also
       the pcrepattern documentation.

         PCRE_EXTENDED

       If this bit is set, whitespace  data  characters  in  the  pattern  are
       totally ignored except when escaped or inside a character class. White-
       space does not include the VT character (code 11). In addition, charac-
       ters between an unescaped # outside a character class and the next new-
       line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
       option,  and  it  can be changed within a pattern by a (?x) option set-
       ting.

       This option makes it possible to include  comments  inside  complicated
       patterns.   Note,  however,  that this applies only to data characters.
       Whitespace  characters  may  never  appear  within  special   character
       sequences  in  a  pattern,  for  example  within the sequence (?( which
       introduces a conditional subpattern.

         PCRE_EXTRA

       This option was invented in order to turn on  additional  functionality
       of  PCRE  that  is  incompatible with Perl, but it is currently of very
       little use. When set, any backslash in a pattern that is followed by  a
       letter  that  has  no  special  meaning causes an error, thus reserving
       these combinations for future expansion. By  default,  as  in  Perl,  a
       backslash  followed by a letter with no special meaning is treated as a
       literal. (Perl can, however, be persuaded to give a warning for  this.)
       There  are  at  present no other features controlled by this option. It
       can also be set by a (?X) option setting within a pattern.

         PCRE_FIRSTLINE

       If this option is set, an  unanchored  pattern  is  required  to  match
       before  or  at  the  first  newline  in  the subject string, though the
       matched text may continue over the newline.

         PCRE_MULTILINE

       By default, PCRE treats the subject string as consisting  of  a  single
       line  of characters (even if it actually contains newlines). The "start
       of line" metacharacter (^) matches only at the  start  of  the  string,
       while  the  "end  of line" metacharacter ($) matches only at the end of
       the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
       is set). This is the same as Perl.

       When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. If there are no new-
       lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
       setting PCRE_MULTILINE has no effect.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF

       These  options  override the default newline definition that was chosen
       when PCRE was built. Setting the first or the second specifies  that  a
       newline  is  indicated  by a single character (CR or LF, respectively).
       Setting both of them specifies that a newline is indicated by the  two-
       character  CRLF sequence. For convenience, PCRE_NEWLINE_CRLF is defined
       to contain both bits. The only time that a line break is relevant  when
       compiling a pattern is if PCRE_EXTENDED is set, and an unescaped # out-
       side a character class is encountered. This indicates  a  comment  that
       lasts until after the next newline.

       The newline option set at compile time becomes the default that is used
       for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

         PCRE_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can  still
       be  used  for  capturing  (and  they acquire numbers in the usual way).
       There is no equivalent of this option in Perl.

         PCRE_UNGREEDY

       This option inverts the "greediness" of the quantifiers  so  that  they
       are  not greedy by default, but become greedy if followed by "?". It is
       not compatible with Perl. It can also be set by a (?U)  option  setting
       within the pattern.

         PCRE_UTF8

       This  option  causes PCRE to regard both the pattern and the subject as
       strings of UTF-8 characters instead of single-byte  character  strings.
       However,  it is available only when PCRE is built to include UTF-8 sup-
       port. If not, the use of this option provokes an error. Details of  how
       this  option  changes the behaviour of PCRE are given in the section on
       UTF-8 support in the main pcre page.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically  checked. If an invalid UTF-8 sequence of bytes is found,
       pcre_compile() returns an error. If you already know that your  pattern
       is  valid, and you want to skip this check for performance reasons, you
       can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
       passing an invalid UTF-8 string as a pattern is undefined. It may cause
       your program to crash.  Note that this option can  also  be  passed  to
       pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
       ing of subject strings.


COMPILATION ERROR CODES

       The following table lists the error  codes  than  may  be  returned  by
       pcre_compile2(),  along with the error messages that may be returned by
       both compiling functions.

          0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  operand of unlimited repeat could match the empty string
         11  internal error: unexpected repeat
         12  unrecognized character after (?
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  parentheses nested too deeply
         20  regular expression too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is not compiled with PCRE_UTF8 support
         33  spare error
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error after (?P
         43  two named subpatterns have the same name
         44  invalid UTF-8 string
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10,000)
         50  repeated subpattern is too long
         51  octal value is greater than \377 (not in UTF-8 mode)


STUDYING A PATTERN

       pcre_extra *pcre_study(const pcre *code, int options
            const char **errptr);

       If a compiled pattern is going to be used several times,  it  is  worth
       spending more time analyzing it in order to speed up the time taken for
       matching. The function pcre_study() takes a pointer to a compiled  pat-
       tern as its first argument. If studying the pattern produces additional
       information that will help speed up matching,  pcre_study()  returns  a
       pointer  to a pcre_extra block, in which the study_data field points to
       the results of the study.

       The  returned  value  from  pcre_study()  can  be  passed  directly  to
       pcre_exec().  However,  a  pcre_extra  block also contains other fields
       that can be set by the caller before the block  is  passed;  these  are
       described below in the section on matching a pattern.

       If  studying  the  pattern  does not produce any additional information
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants  to  pass  any of the other fields to pcre_exec(), it must set up
       its own pcre_extra block.

       The second argument of pcre_study() contains option bits.  At  present,
       no options are defined, and this argument should always be zero.

       The  third argument for pcre_study() is a pointer for an error message.
       If studying succeeds (even if no data is  returned),  the  variable  it
       points  to  is  set  to NULL. Otherwise it is set to point to a textual
       error message. This is a static string that is part of the library. You
       must  not  try  to  free it. You should test the error pointer for NULL
       after calling pcre_study(), to be sure that it has run successfully.

       This is a typical call to pcre_study():

         pcre_extra *pe;
         pe = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options exist */
           &error);        /* set to NULL or points to a message */

       At present, studying a pattern is useful only for non-anchored patterns
       that  do not have a single fixed starting character. A bitmap of possi-
       ble starting bytes is created.


LOCALE SUPPORT

       PCRE handles caseless matching, and determines whether  characters  are
       letters  digits,  or whatever, by reference to a set of tables, indexed
       by character value. When running in UTF-8 mode, this  applies  only  to
       characters  with  codes  less than 128. Higher-valued codes never match
       escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
       with  Unicode  character property support. The use of locales with Uni-
       code is discouraged.

       An internal set of tables is created in the default C locale when  PCRE
       is  built.  This  is  used when the final argument of pcre_compile() is
       NULL, and is sufficient for many applications. An  alternative  set  of
       tables  can,  however, be supplied. These may be created in a different
       locale from the default. As more and more applications change to  using
       Unicode, the need for this locale support is expected to die away.

       External  tables  are  built by calling the pcre_maketables() function,
       which has no arguments, in the relevant locale. The result can then  be
       passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
       example, to build and use tables that are appropriate  for  the  French
       locale  (where  accented  characters  with  values greater than 128 are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre_maketables();
         re = pcre_compile(..., tables);

       When pcre_maketables() runs, the tables are built  in  memory  that  is
       obtained  via  pcre_malloc. It is the caller's responsibility to ensure
       that the memory containing the tables remains available for as long  as
       it is needed.

       The pointer that is passed to pcre_compile() is saved with the compiled
       pattern, and the same tables are used via this pointer by  pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

       It  is  possible to pass a table pointer or NULL (indicating the use of
       the internal tables) to pcre_exec(). Although  not  intended  for  this
       purpose,  this facility could be used to match a pattern in a different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.


INFORMATION ABOUT A PATTERN

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       The  pcre_fullinfo() function returns information about a compiled pat-
       tern. It replaces the obsolete pcre_info() function, which is neverthe-
       less retained for backwards compability (and is documented below).

       The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
       pattern. The second argument is the result of pcre_study(), or NULL  if
       the  pattern  was not studied. The third argument specifies which piece
       of information is required, and the fourth argument is a pointer  to  a
       variable  to  receive  the  data. The yield of the function is zero for
       success, or one of the following negative numbers:

         PCRE_ERROR_NULL       the argument code was NULL
                               the argument where was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found
         PCRE_ERROR_BADOPTION  the value of what was invalid

       The "magic number" is placed at the start of each compiled  pattern  as
       an  simple check against passing an arbitrary memory pointer. Here is a
       typical call of pcre_fullinfo(), to obtain the length of  the  compiled
       pattern:

         int rc;
         size_t length;
         rc = pcre_fullinfo(
           re,               /* result of pcre_compile() */
           pe,               /* result of pcre_study(), or NULL */
           PCRE_INFO_SIZE,   /* what is required */
           &length);         /* where to put the data */

       The  possible  values for the third argument are defined in pcre.h, and
       are as follows:

         PCRE_INFO_BACKREFMAX

       Return the number of the highest back reference  in  the  pattern.  The
       fourth  argument  should  point to an int variable. Zero is returned if
       there are no back references.

         PCRE_INFO_CAPTURECOUNT

       Return the number of capturing subpatterns in the pattern.  The  fourth
       argument should point to an int variable.

         PCRE_INFO_DEFAULT_TABLES

       Return  a pointer to the internal default character tables within PCRE.
       The fourth argument should point to an unsigned char *  variable.  This
       information call is provided for internal use by the pcre_study() func-
       tion. External callers can cause PCRE to use  its  internal  tables  by
       passing a NULL table pointer.

         PCRE_INFO_FIRSTBYTE

       Return  information  about  the first byte of any matched string, for a
       non-anchored pattern. The fourth argument should point to an int  vari-
       able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
       is still recognized for backwards compatibility.)

       If there is a fixed first byte, for example, from  a  pattern  such  as
       (cat|cow|coyote). Otherwise, if either

       (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
       branch starts with "^", or

       (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
       set (if it were set, the pattern would be anchored),

       -1  is  returned, indicating that the pattern matches only at the start
       of a subject string or after any newline within the  string.  Otherwise
       -2 is returned. For anchored patterns, -2 is returned.

         PCRE_INFO_FIRSTTABLE

       If  the pattern was studied, and this resulted in the construction of a
       256-bit table indicating a fixed set of bytes for the first byte in any
       matching  string, a pointer to the table is returned. Otherwise NULL is
       returned. The fourth argument should point to an unsigned char *  vari-
       able.

         PCRE_INFO_LASTLITERAL

       Return  the  value of the rightmost literal byte that must exist in any
       matched string, other than at its  start,  if  such  a  byte  has  been
       recorded. The fourth argument should point to an int variable. If there
       is no such byte, -1 is returned. For anchored patterns, a last  literal
       byte  is  recorded only if it follows something of variable length. For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.

         PCRE_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

       PCRE  supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre_get_named_substring() are provided for  extracting  captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers in the output vector (described with pcre_exec() below). To do
       the conversion, you need  to  use  the  name-to-number  map,  which  is
       described by these three values.

       The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of  each  entry;  both  of  these  return  an int value. The entry size
       depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
       a  pointer  to  the  first  entry of the table (a pointer to char). The
       first two bytes of each entry are the number of the capturing parenthe-
       sis,  most  significant byte first. The rest of the entry is the corre-
       sponding name, zero terminated. The names are  in  alphabetical  order.
       When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
       theses numbers. For example, consider  the  following  pattern  (assume
       PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
       ignored):

         (?P<date> (?P<year>(\d\d)?\d\d) -
         (?P<month>\d\d) - (?P<day>\d\d) )

       There are four named subpatterns, so the table has  four  entries,  and
       each  entry  in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When  writing  code  to  extract  data from named subpatterns using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE_INFO_OPTIONS

       Return  a  copy of the options with which the pattern was compiled. The
       fourth argument should point to an unsigned long  int  variable.  These
       option bits are those specified in the call to pcre_compile(), modified
       by any top-level option settings within the pattern itself.

       A pattern is automatically anchored by PCRE if  all  of  its  top-level
       alternatives begin with one of the following:

         ^     unless PCRE_MULTILINE is set
         \A    always
         \G    always
         .*    if PCRE_DOTALL is set and there are no back
                 references to the subpattern in which .* appears

       For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().

         PCRE_INFO_SIZE

       Return the size of the compiled pattern, that is, the  value  that  was
       passed as the argument to pcre_malloc() when PCRE was getting memory in
       which to place the compiled data. The fourth argument should point to a
       size_t variable.

         PCRE_INFO_STUDYSIZE

       Return the size of the data block pointed to by the study_data field in
       a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
       pcre_malloc() when PCRE was getting memory into which to place the data
       created by pcre_study(). The fourth argument should point to  a  size_t
       variable.


OBSOLETE INFO FUNCTION

       int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

       The  pcre_info()  function is now obsolete because its interface is too
       restrictive to return all the available data about a compiled  pattern.
       New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
       pcre_info() is the number of capturing subpatterns, or one of the  fol-
       lowing negative numbers:

         PCRE_ERROR_NULL       the argument code was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found

       If  the  optptr  argument is not NULL, a copy of the options with which
       the pattern was compiled is placed in the integer  it  points  to  (see
       PCRE_INFO_OPTIONS above).

       If  the  pattern  is  not anchored and the firstcharptr argument is not
       NULL, it is used to pass back information about the first character  of
       any matched string (see PCRE_INFO_FIRSTBYTE above).


REFERENCE COUNTS

       int pcre_refcount(pcre *code, int adjust);

       The  pcre_refcount()  function is used to maintain a reference count in
       the data block that contains a compiled pattern. It is provided for the
       benefit  of  applications  that  operate  in an object-oriented manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

       When a pattern is compiled, the reference count field is initialized to
       zero.  It is changed only by calling this function, whose action is  to
       add  the  adjust  value  (which may be positive or negative) to it. The
       yield of the function is the new value. However, the value of the count
       is  constrained to lie between 0 and 65535, inclusive. If the new value
       is outside these limits, it is forced to the appropriate limit value.

       Except when it is zero, the reference count is not correctly  preserved
       if  a  pattern  is  compiled on one host and then transferred to a host
       whose byte-order is different. (This seems a highly unlikely scenario.)


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       The  function pcre_exec() is called to match a subject string against a
       compiled pattern, which is passed in the code argument. If the  pattern
       has been studied, the result of the study should be passed in the extra
       argument. This function is the main matching facility of  the  library,
       and it operates in a Perl-like manner. For specialist use there is also
       an alternative matching function, which is described below in the  sec-
       tion about the pcre_dfa_exec() function.

       In  most applications, the pattern will have been compiled (and option-
       ally studied) in the same process that calls pcre_exec().  However,  it
       is possible to save compiled patterns and study data, and then use them
       later in different processes, possibly even on different hosts.  For  a
       discussion about this, see the pcreprecompile documentation.

       Here is an example of a simple call to pcre_exec():

         int rc;
         int ovector[30];
         rc = pcre_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

   Extra data for pcre_exec()

       If  the  extra argument is not NULL, it must point to a pcre_extra data
       block. The pcre_study() function returns such a block (when it  doesn't
       return  NULL), but you can also create one for yourself, and pass addi-
       tional information in it. The pcre_extra block contains  the  following
       fields (not necessarily in this order):

         unsigned long int flags;
         void *study_data;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;

       The  flags  field  is a bitmap that specifies which of the other fields
       are set. The flag bits are:

         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_TABLES

       Other flag bits should be set to zero. The study_data field is  set  in
       the  pcre_extra  block  that is returned by pcre_study(), together with
       the appropriate flag bit. You should not set this yourself, but you may
       add  to  the  block by setting the other fields and their corresponding
       flag bits.

       The match_limit field provides a means of preventing PCRE from using up
       a  vast amount of resources when running patterns that are not going to
       match, but which have a very large number  of  possibilities  in  their
       search  trees.  The  classic  example  is  the  use of nested unlimited
       repeats.

       Internally, PCRE uses a function called match() which it calls  repeat-
       edly  (sometimes  recursively). The limit set by match_limit is imposed
       on the number of times this function is called during  a  match,  which
       has  the  effect  of  limiting the amount of backtracking that can take
       place. For patterns that are not anchored, the count restarts from zero
       for each position in the subject string.

       The  default  value  for  the  limit can be set when PCRE is built; the
       default default is 10 million, which handles all but the  most  extreme
       cases.  You  can  override  the  default by suppling pcre_exec() with a
       pcre_extra    block    in    which    match_limit    is    set,     and
       PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

       The match_limit_recursion field is similar to match_limit, but  instead
       of limiting the total number of times that match() is called, it limits
       the depth of recursion. The recursion depth is a  smaller  number  than
       the  total number of calls, because not all calls to match() are recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting  the  recursion  depth  limits the amount of stack that can be
       used, or, when PCRE has been compiled to use memory on the heap instead
       of the stack, the amount of heap memory that can be used.

       The  default  value  for  match_limit_recursion can be set when PCRE is
       built; the default default  is  the  same  value  as  the  default  for
       match_limit.  You can override the default by suppling pcre_exec() with
       a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

       The pcre_callout field is used in conjunction with the  "callout"  fea-
       ture, which is described in the pcrecallout documentation.

       The  tables  field  is  used  to  pass  a  character  tables pointer to
       pcre_exec(); this overrides the value that is stored with the  compiled
       pattern.  A  non-NULL value is stored with the compiled pattern only if
       custom tables were supplied to pcre_compile() via  its  tableptr  argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's internal tables to be used. This facility is  helpful  when  re-
       using  patterns  that  have been saved after compiling with an external
       set of tables, because the external tables  might  be  at  a  different
       address  when  pcre_exec() is called. See the pcreprecompile documenta-
       tion for a discussion of saving compiled patterns for later use.

   Option bits for pcre_exec()

       The unused bits of the options argument for pcre_exec() must  be  zero.
       The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
       PCRE_PARTIAL.

         PCRE_ANCHORED

       The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
       matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
       turned  out to be anchored by virtue of its contents, it cannot be made
       unachored at matching time.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF

       These options override  the  newline  definition  that  was  chosen  or
       defaulted  when the pattern was compiled. For details, see the descrip-
       tion pcre_compile() above. During matching, the newline choice  affects
       the behaviour of the dot, circumflex, and dollar metacharacters.

         PCRE_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before it. Setting this without PCRE_MULTILINE (at compile time)
       causes circumflex never to match. This option affects only  the  behav-
       iour of the circumflex metacharacter. It does not affect \A.

         PCRE_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option  affects only the behaviour of the dollar metacharacter. It does
       not affect \Z or \z.

         PCRE_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a string not beginning with "a" or "b", it matches the
       empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
       match is not valid, so PCRE searches further into the string for occur-
       rences of "a" or "b".

       Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
       cial  case  of  a  pattern match of the empty string within its split()
       function, and when using the /g modifier. It  is  possible  to  emulate
       Perl's behaviour after matching a null string by first trying the match
       again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
       if  that  fails by advancing the starting offset (see below) and trying
       an ordinary match again. There is some code that demonstrates how to do
       this in the pcredemo.c sample program.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8 string is automatically checked when pcre_exec() is  subsequently
       called.   The  value  of  startoffset is also checked to ensure that it
       points to the start of a UTF-8 character. If an invalid UTF-8  sequence
       of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
       startoffset contains an  invalid  value,  PCRE_ERROR_BADUTF8_OFFSET  is
       returned.

       If  you  already  know that your subject is valid, and you want to skip
       these   checks   for   performance   reasons,   you   can    set    the
       PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
       do this for the second and subsequent calls to pcre_exec() if  you  are
       making  repeated  calls  to  find  all  the matches in a single subject
       string. However, you should be  sure  that  the  value  of  startoffset
       points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
       set, the effect of passing an invalid UTF-8 string as a subject,  or  a
       value  of startoffset that does not point to the start of a UTF-8 char-
       acter, is undefined. Your program may crash.

         PCRE_PARTIAL

       This option turns on the  partial  matching  feature.  If  the  subject
       string  fails to match the pattern, but at some point during the match-
       ing process the end of the subject was reached (that  is,  the  subject
       partially  matches  the  pattern and the failure to match occurred only
       because there were not enough subject characters), pcre_exec()  returns
       PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
       used, there are restrictions on what may appear in the  pattern.  These
       are discussed in the pcrepartial documentation.

   The string to be matched by pcre_exec()

       The  subject string is passed to pcre_exec() as a pointer in subject, a
       length in length, and a starting byte offset in startoffset.  In  UTF-8
       mode,  the  byte  offset  must point to the start of a UTF-8 character.
       Unlike the pattern string, the subject may contain binary  zero  bytes.
       When  the starting offset is zero, the search for a match starts at the
       beginning of the subject, and this is by far the most common case.

       A non-zero starting offset is useful when searching for  another  match
       in  the same subject by calling pcre_exec() again after a previous suc-
       cess.  Setting startoffset differs from just passing over  a  shortened
       string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which finds occurrences of "iss" in the middle of  words.  (\B  matches
       only  if  the  current position in the subject is not a word boundary.)
       When applied to the string "Mississipi" the first call  to  pcre_exec()
       finds  the  first  occurrence. If pcre_exec() is called again with just
       the remainder of the subject,  namely  "issipi",  it  does  not  match,
       because \B is always false at the start of the subject, which is deemed
       to be a word boundary. However, if pcre_exec()  is  passed  the  entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence of "iss" because it is able to look behind the starting point  to
       discover that it is preceded by a letter.

       If  a  non-zero starting offset is passed when the pattern is anchored,
       one attempt to match at the given offset is made. This can only succeed
       if  the  pattern  does  not require the match to be at the start of the
       subject.

   How pcre_exec() returns captured substrings

       In general, a pattern matches a certain portion of the subject, and  in
       addition,  further  substrings  from  the  subject may be picked out by
       parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
       this  is  called "capturing" in what follows, and the phrase "capturing
       subpattern" is used for a fragment of a pattern that picks out  a  sub-
       string.  PCRE  supports several other kinds of parenthesized subpattern
       that do not cause substrings to be captured.

       Captured substrings are returned to the caller via a vector of  integer
       offsets  whose  address is passed in ovector. The number of elements in
       the vector is passed in ovecsize, which must be a non-negative  number.
       Note: this argument is NOT the size of ovector in bytes.

       The  first  two-thirds of the vector is used to pass back captured sub-
       strings, each substring using a pair of integers. The  remaining  third
       of  the  vector is used as workspace by pcre_exec() while matching cap-
       turing subpatterns, and is not available for passing back  information.
       The  length passed in ovecsize should always be a multiple of three. If
       it is not, it is rounded down.

       When a match is successful, information about  captured  substrings  is
       returned  in  pairs  of integers, starting at the beginning of ovector,
       and continuing up to two-thirds of its length at the  most.  The  first
       element of a pair is set to the offset of the first character in a sub-
       string, and the second is set to the  offset  of  the  first  character
       after  the  end  of  a  substring. The first pair, ovector[0] and ovec-
       tor[1], identify the portion of  the  subject  string  matched  by  the
       entire  pattern.  The next pair is used for the first capturing subpat-
       tern, and so on. The value returned by pcre_exec() is one more than the
       highest numbered pair that has been set. For example, if two substrings
       have been captured, the returned value is 3. If there are no  capturing
       subpatterns,  the return value from a successful match is 1, indicating
       that just the first pair of offsets has been set.

       If a capturing subpattern is matched repeatedly, it is the last portion
       of the string that it matched that is returned.

       If  the vector is too small to hold all the captured substring offsets,
       it is used as far as possible (up to two-thirds of its length), and the
       function  returns a value of zero. In particular, if the substring off-
       sets are not of interest, pcre_exec() may be called with ovector passed
       as  NULL  and  ovecsize  as zero. However, if the pattern contains back
       references and the ovector is not big enough to  remember  the  related
       substrings,  PCRE has to get additional memory for use during matching.
       Thus it is usually advisable to supply an ovector.

       The pcre_info() function can be used to find  out  how  many  capturing
       subpatterns  there  are  in  a  compiled pattern. The smallest size for
       ovector that will allow for n captured substrings, in addition  to  the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

       It  is  possible for capturing subpattern number n+1 to match some part
       of the subject when subpattern n has not been used at all. For example,
       if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2  is  not.  When  this happens, both values in the offset pairs corre-
       sponding to unused subpatterns are set to -1.

       Offset values that correspond to unused subpatterns at the end  of  the
       expression  are  also  set  to  -1. For example, if the string "abc" is
       matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not
       matched.  The  return  from the function is 2, because the highest used
       capturing subpattern number is 1. However, you can refer to the offsets
       for  the  second  and third capturing subpatterns if you wish (assuming
       the vector is large enough, of course).

       Some convenience functions are provided  for  extracting  the  captured
       substrings as separate strings. These are described below.

   Error return values from pcre_exec()

       If  pcre_exec()  fails, it returns a negative number. The following are
       defined in the header file:

         PCRE_ERROR_NOMATCH        (-1)

       The subject string did not match the pattern.

         PCRE_ERROR_NULL           (-2)

       Either code or subject was passed as NULL,  or  ovector  was  NULL  and
       ovecsize was not zero.

         PCRE_ERROR_BADOPTION      (-3)

       An unrecognized bit was set in the options argument.

         PCRE_ERROR_BADMAGIC       (-4)

       PCRE  stores a 4-byte "magic number" at the start of the compiled code,
       to catch the case when it is passed a junk pointer and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an environment with the other endianness. This is the error  that  PCRE
       gives when the magic number is not present.

         PCRE_ERROR_UNKNOWN_NODE   (-5)

       While running the pattern match, an unknown item was encountered in the
       compiled pattern. This error could be caused by a bug  in  PCRE  or  by
       overwriting of the compiled pattern.

         PCRE_ERROR_NOMEMORY       (-6)

       If  a  pattern contains back references, but the ovector that is passed
       to pcre_exec() is not big enough to remember the referenced substrings,
       PCRE  gets  a  block of memory at the start of matching to use for this
       purpose. If the call via pcre_malloc() fails, this error is given.  The
       memory is automatically freed at the end of matching.

         PCRE_ERROR_NOSUBSTRING    (-7)

       This  error is used by the pcre_copy_substring(), pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see  below).  It  is  never
       returned by pcre_exec().

         PCRE_ERROR_MATCHLIMIT     (-8)

       The  backtracking  limit,  as  specified  by the match_limit field in a
       pcre_extra structure (or defaulted) was reached.  See  the  description
       above.

         PCRE_ERROR_RECURSIONLIMIT (-21)

       The internal recursion limit, as specified by the match_limit_recursion
       field in a pcre_extra structure (or defaulted)  was  reached.  See  the
       description above.

         PCRE_ERROR_CALLOUT        (-9)

       This error is never generated by pcre_exec() itself. It is provided for
       use by callout functions that want to yield a distinctive  error  code.
       See the pcrecallout documentation for details.

         PCRE_ERROR_BADUTF8        (-10)

       A  string  that contains an invalid UTF-8 byte sequence was passed as a
       subject.

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       The UTF-8 byte sequence that was passed as a subject was valid, but the
       value  of startoffset did not point to the beginning of a UTF-8 charac-
       ter.

         PCRE_ERROR_PARTIAL        (-12)

       The subject string did not match, but it did match partially.  See  the
       pcrepartial documentation for details of partial matching.

         PCRE_ERROR_BADPARTIAL     (-13)

       The  PCRE_PARTIAL  option  was  used with a compiled pattern containing
       items that are not supported for partial matching. See the  pcrepartial
       documentation for details of partial matching.

         PCRE_ERROR_INTERNAL       (-14)

       An  unexpected  internal error has occurred. This error could be caused
       by a bug in PCRE or by overwriting of the compiled pattern.

         PCRE_ERROR_BADCOUNT       (-15)

       This error is given if the value of the ovecsize argument is  negative.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       Captured  substrings  can  be  accessed  directly  by using the offsets
       returned by pcre_exec() in  ovector.  For  convenience,  the  functions
       pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
       string_list() are provided for extracting captured substrings  as  new,
       separate,  zero-terminated strings. These functions identify substrings
       by number. The next section describes functions  for  extracting  named
       substrings.

       A  substring that contains a binary zero is correctly extracted and has
       a further zero added on the end, but the result is not, of course, a  C
       string.   However,  you  can  process such a string by referring to the
       length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not adequate for handling strings containing binary zeros, because  the
       end of the final string is not independently indicated.

       The  first  three  arguments  are the same for all three of these func-
       tions: subject is the subject string that has  just  been  successfully
       matched, ovector is a pointer to the vector of integer offsets that was
       passed to pcre_exec(), and stringcount is the number of substrings that
       were  captured  by  the match, including the substring that matched the
       entire regular expression. This is the value returned by pcre_exec() if
       it  is greater than zero. If pcre_exec() returned zero, indicating that
       it ran out of space in ovector, the value passed as stringcount  should
       be the number of elements in the vector divided by three.

       The  functions pcre_copy_substring() and pcre_get_substring() extract a
       single substring, whose number is given as  stringnumber.  A  value  of
       zero  extracts  the  substring that matched the entire pattern, whereas
       higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
       string(),  the  string  is  placed  in buffer, whose length is given by
       buffersize, while for pcre_get_substring() a new  block  of  memory  is
       obtained  via  pcre_malloc,  and its address is returned via stringptr.
       The yield of the function is the length of the  string,  not  including
       the terminating zero, or one of

         PCRE_ERROR_NOMEMORY       (-6)

       The  buffer  was too small for pcre_copy_substring(), or the attempt to
       get memory failed for pcre_get_substring().

         PCRE_ERROR_NOSUBSTRING    (-7)

       There is no substring whose number is stringnumber.

       The pcre_get_substring_list()  function  extracts  all  available  sub-
       strings  and  builds  a list of pointers to them. All this is done in a
       single block of memory that is obtained via pcre_malloc. The address of
       the  memory  block  is returned via listptr, which is also the start of
       the list of string pointers. The end of the list is marked  by  a  NULL
       pointer. The yield of the function is zero if all went well, or

         PCRE_ERROR_NOMEMORY       (-6)

       if the attempt to get the memory block failed.

       When  any of these functions encounter a substring that is unset, which
       can happen when capturing subpattern number n+1 matches  some  part  of
       the  subject, but subpattern n has not been used at all, they return an
       empty string. This can be distinguished from a genuine zero-length sub-
       string  by inspecting the appropriate offset in ovector, which is nega-
       tive for unset substrings.

       The two convenience functions pcre_free_substring() and  pcre_free_sub-
       string_list()  can  be  used  to free the memory returned by a previous
       call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
       tively.  They  do  nothing  more  than  call the function pointed to by
       pcre_free, which of course could be called directly from a  C  program.
       However,  PCRE is used in some situations where it is linked via a spe-
       cial  interface  to  another  programming  language  that  cannot   use
       pcre_free  directly;  it is for these cases that the functions are pro-
       vided.


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       To extract a substring by name, you first have to find associated  num-
       ber.  For example, for this pattern

         (a+)b(?P<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
       subpattern of that name.

       Given the number, you can extract the substring directly, or use one of
       the functions described in the previous section. For convenience, there
       are also two functions that do the whole job.

       Most   of   the   arguments    of    pcre_copy_named_substring()    and
       pcre_get_named_substring()  are  the  same  as  those for the similarly
       named functions that extract by number. As these are described  in  the
       previous  section,  they  are not re-described here. There are just two
       differences:

       First, instead of a substring number, a substring name is  given.  Sec-
       ond, there is an extra argument, given at the start, which is a pointer
       to the compiled pattern. This is needed in order to gain access to  the
       name-to-number translation table.

       These  functions call pcre_get_stringnumber(), and if it succeeds, they
       then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
       ate.


DUPLICATE SUBPATTERN NAMES

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
       subpatterns are not required to  be  unique.  Normally,  patterns  with
       duplicate  names  are such that in any one match, only one of the named
       subpatterns participates. An example is shown in the pcrepattern  docu-
       mentation. When duplicates are present, pcre_copy_named_substring() and
       pcre_get_named_substring() return the first substring corresponding  to
       the  given  name  that  is  set.  If  none  are set, an empty string is
       returned.  The pcre_get_stringnumber() function returns one of the num-
       bers  that are associated with the name, but it is not defined which it
       is.

       If you want to get full details of all captured substrings for a  given
       name,  you  must  use  the pcre_get_stringtable_entries() function. The
       first argument is the compiled pattern, and the second is the name. The
       third  and  fourth  are  pointers to variables which are updated by the
       function. After it has run, they point to the first and last entries in
       the  name-to-number  table  for  the  given  name.  The function itself
       returns the length of each entry, or  PCRE_ERROR_NOSUBSTRING  if  there
       are  none.  The  format  of the table is described above in the section
       entitled Information about a pattern. Given all  the  relevant  entries
       for the name, you can extract each of their numbers, and hence the cap-
       tured data, if any.


FINDING ALL POSSIBLE MATCHES

       The traditional matching function uses a  similar  algorithm  to  Perl,
       which stops when it finds the first match, starting at a given point in
       the subject. If you want to find all possible matches, or  the  longest
       possible  match,  consider using the alternative matching function (see
       below) instead. If you cannot use the alternative function,  but  still
       need  to  find all possible matches, you can kludge it up by making use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.  When your callout function is called, extract and save the  cur-
       rent  matched  substring.  Then  return  1, which forces pcre_exec() to
       backtrack and try other alternatives. Ultimately, when it runs  out  of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       The  function  pcre_dfa_exec()  is  called  to  match  a subject string
       against a compiled pattern, using a "DFA" matching algorithm. This  has
       different  characteristics to the normal algorithm, and is not compati-
       ble with Perl. Some of the features of PCRE patterns are not supported.
       Nevertheless, there are times when this kind of matching can be useful.
       For a discussion of the two matching algorithms, see  the  pcrematching
       documentation.

       The  arguments  for  the  pcre_dfa_exec()  function are the same as for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent  way,  and  this is described below. The other common arguments are
       used in the same way as for pcre_exec(), so their  description  is  not
       repeated here.

       The  two  additional  arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It  is  used  for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace will be needed for patterns and subjects where  there  are  a
       lot of potential matches.

       Here is an example of a simple call to pcre_dfa_exec():

         int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_exec()

       The  unused  bits  of  the options argument for pcre_dfa_exec() must be
       zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
       LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
       PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
       three of these are the same as for pcre_exec(), so their description is
       not repeated here.

         PCRE_PARTIAL

       This has the same general effect as it does for  pcre_exec(),  but  the
       details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
       pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
       PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
       been no complete matches, but there is still at least one matching pos-
       sibility.  The portion of the string that provided the partial match is
       set as the first matching string.

         PCRE_DFA_SHORTEST

       Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
       stop  as  soon  as  it  has found one match. Because of the way the DFA
       algorithm works, this is necessarily the shortest possible match at the
       first possible matching point in the subject string.

         PCRE_DFA_RESTART

       When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
       returns a partial match, it is possible to call it  again,  with  addi-
       tional  subject  characters,  and have it continue with the same match.
       The PCRE_DFA_RESTART option requests this action; when it is  set,  the
       workspace  and wscount options must reference the same vector as before
       because data about the match so far is left in  them  after  a  partial
       match.  There  is  more  discussion of this facility in the pcrepartial
       documentation.

   Successful returns from pcre_dfa_exec()

       When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
       string in the subject. Note, however, that all the matches from one run
       of the function start at the same point in  the  subject.  The  shorter
       matches  are all initial substrings of the longer matches. For example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something>
         <something> <something else>
         <something> <something else> <something further>

       On success, the yield of the function is a number  greater  than  zero,
       which  is  the  number of matched substrings. The substrings themselves
       are returned in ovector. Each string uses two elements;  the  first  is
       the  offset  to the start, and the second is the offset to the end. All
       the strings have the same start offset. (Space could have been saved by
       giving  this only once, but it was decided to retain some compatibility
       with the way pcre_exec() returns data, even though the meaning  of  the
       strings is different.)

       The strings are returned in reverse order of length; that is, the long-
       est matching string is given first. If there were too many  matches  to
       fit  into ovector, the yield of the function is zero, and the vector is
       filled with the longest matches.

   Error returns from pcre_dfa_exec()

       The pcre_dfa_exec() function returns a negative number when  it  fails.
       Many  of  the  errors  are  the  same as for pcre_exec(), and these are
       described above.  There are in addition the following errors  that  are
       specific to pcre_dfa_exec():

         PCRE_ERROR_DFA_UITEM      (-16)

       This  return is given if pcre_dfa_exec() encounters an item in the pat-
       tern that it does not support, for instance, the use of \C  or  a  back
       reference.

         PCRE_ERROR_DFA_UCOND      (-17)

       This  return is given if pcre_dfa_exec() encounters a condition item in
       a pattern that uses a back reference for the  condition.  This  is  not
       supported.

         PCRE_ERROR_DFA_UMLIMIT    (-18)

       This  return  is given if pcre_dfa_exec() is called with an extra block
       that contains a setting of the match_limit field. This is not supported
       (it is meaningless).

         PCRE_ERROR_DFA_WSSIZE     (-19)

       This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
       workspace vector.

         PCRE_ERROR_DFA_RECURSE    (-20)

       When a recursive subpattern is processed, the matching  function  calls
       itself  recursively,  using  private vectors for ovector and workspace.
       This error is given if the output vector  is  not  large  enough.  This
       should be extremely rare, as a vector of size 1000 is used.

Last updated: 08 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCRECALLOUT(3)                                                  PCRECALLOUT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout. By  default,  this
       variable contains NULL, which disables all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called.  Different  callout  points  can  be
       identified  by  putting  a number less than 256 after the letter C. The
       default value is zero.  For  example,  this  pattern  has  two  callout
       points:

         (?C1)eabc(?C2)def

       If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
       called, PCRE automatically  inserts  callouts,  all  with  number  255,
       before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
       used with the pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice that there is a callout before and after  each  parenthesis  and
       alternation  bar.  Automatic  callouts  can  be  used  for tracking the
       progress of pattern matching. The pcretest command has an  option  that
       sets  automatic callouts; when it is used, the output indicates how the
       pattern is matched. This is useful information when you are  trying  to
       optimize the performance of a particular pattern.


MISSING CALLOUTS

       You  should  be  aware  that,  because of optimizations in the way PCRE
       matches patterns, callouts sometimes do not happen. For example, if the
       pattern is

         ab(?C4)cd

       PCRE knows that any matching string must contain the letter "d". If the
       subject string is "abyz", the lack of "d" means that  matching  doesn't
       ever  start,  and  the  callout is never reached. However, with "abyd",
       though the result is still no match, the callout is obeyed.


THE CALLOUT INTERFACE

       During matching, when PCRE reaches a callout point, the external  func-
       tion  defined by pcre_callout is called (if it is set). This applies to
       both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
       only  argument  to  the callout function is a pointer to a pcre_callout
       block. This structure contains the following fields:

         int          version;
         int          callout_number;
         int         *offset_vector;
         const char  *subject;
         int          subject_length;
         int          start_match;
         int          current_position;
         int          capture_top;
         int          capture_last;
         void        *callout_data;
         int          pattern_position;
         int          next_item_length;

       The version field is an integer containing the version  number  of  the
       block  format. The initial version was 0; the current version is 1. The
       version number will change again in future  if  additional  fields  are
       added, but the intention is never to remove any of the existing fields.

       The callout_number field contains the number of the  callout,  as  com-
       piled  into  the pattern (that is, the number after ?C for manual call-
       outs, and 255 for automatically generated callouts).

       The offset_vector field is a pointer to the vector of offsets that  was
       passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
       pcre_exec() is used, the contents can be inspected in order to  extract
       substrings  that  have  been  matched  so  far,  in the same way as for
       extracting substrings after a match has completed. For  pcre_dfa_exec()
       this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to pcre_exec().

       The start_match field contains the offset within the subject  at  which
       the  current match attempt started. If the pattern is not anchored, the
       callout function may be called several times from the same point in the
       pattern for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       When the pcre_exec() function is used, the capture_top  field  contains
       one  more than the number of the highest numbered captured substring so
       far. If no substrings have been captured, the value of  capture_top  is
       one.  This  is always the case when pcre_dfa_exec() is used, because it
       does not support captured substrings.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring. If no substrings have been captured, its value is -1.
       This is always the case when pcre_dfa_exec() is used.

       The callout_data field contains a value that is passed  to  pcre_exec()
       or  pcre_dfa_exec() specifically so that it can be passed back in call-
       outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
       structure.  If  no such data was passed, the value of callout_data in a
       pcre_callout block is NULL. There is a description  of  the  pcre_extra
       structure in the pcreapi documentation.

       The  pattern_position field is present from version 1 of the pcre_call-
       out structure. It contains the offset to the next item to be matched in
       the pattern string.

       The  next_item_length field is present from version 1 of the pcre_call-
       out structure. It contains the length of the next item to be matched in
       the  pattern  string. When the callout immediately precedes an alterna-
       tion bar, a closing parenthesis, or the end of the pattern, the  length
       is  zero.  When the callout precedes an opening parenthesis, the length
       is that of the entire subpattern.

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts.


RETURN VALUES

       The external callout function returns an integer to PCRE. If the  value
       is  zero,  matching  proceeds  as  normal. If the value is greater than
       zero, matching fails at the current point, but  the  testing  of  other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  and
       pcre_exec() (or pcre_dfa_exec()) returns the negative value.

       Negative   values   should   normally   be   chosen  from  the  set  of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be  used  by  PCRE
       itself.

Last updated: 28 February 2005
Copyright (c) 1997-2005 University of Cambridge.
------------------------------------------------------------------------------


PCRECOMPAT(3)                                                    PCRECOMPAT(3)


NAME
       PCRE - Perl-compatible regular expressions


DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl 5.8.

       1.  PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
       of what it does have are given in the section on UTF-8 support  in  the
       main pcre page.

       2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
       permits them, but they do not mean what you might think.  For  example,
       (?!a){3} does not assert that the next three characters are not "a". It
       just asserts that the next character is not "a" three times.

       3. Capturing subpatterns that occur inside  negative  lookahead  asser-
       tions  are  counted,  but their entries in the offsets vector are never
       set. Perl sets its numerical variables from any such patterns that  are
       matched before the assertion fails to match something (thereby succeed-
       ing), but only if the negative lookahead assertion  contains  just  one
       branch.

       4.  Though  binary zero characters are supported in the subject string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5. The following Perl escape sequences are not supported: \l,  \u,  \L,
       \U, and \N. In fact these are implemented by Perl's general string-han-
       dling and are not part of its pattern matching engine. If any of  these
       are encountered by PCRE, an error is generated.

       6.  The Perl escape sequences \p, \P, and \X are supported only if PCRE
       is built with Unicode character property support. The  properties  that
       can  be tested with \p and \P are limited to the general category prop-
       erties such as Lu and Nd, script names such as Greek or  Han,  and  the
       derived properties Any and L&.

       7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters in between are treated as literals.  This  is  slightly  different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course  PCRE
       does not have variables). Note the following examples:

           Pattern            PCRE matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
       constructions.  However,  there is support for recursive patterns using
       the non-Perl items (?R),  (?number),  and  (?P>name).  Also,  the  PCRE
       "callout"  feature allows an external function to be called during pat-
       tern matching. See the pcrecallout documentation for details.

       9. There are some differences that are concerned with the  settings  of
       captured  strings  when  part  of  a  pattern is repeated. For example,
       matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
       unset, but in PCRE it is set to "b".

       10. PCRE provides some extensions to the Perl regular expression facil-
       ities:

       (a) Although lookbehind assertions must  match  fixed  length  strings,
       each alternative branch of a lookbehind assertion can match a different
       length of string. Perl requires them all to have the same length.

       (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
       meta-character matches only at the very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
       cial meaning  is  faulted.  Otherwise,  like  Perl,  the  backslash  is
       ignored. (Perl can be made to issue a warning.)

       (d)  If  PCRE_UNGREEDY is set, the greediness of the repetition quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

       (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and  PCRE_NO_AUTO_CAP-
       TURE options for pcre_exec() have no Perl equivalents.

       (g)  The (?R), (?number), and (?P>name) constructs allows for recursive
       pattern matching (Perl can do  this  using  the  (?p{code})  construct,
       which PCRE cannot support.)

       (h)  PCRE supports named capturing substrings, using the Python syntax.

       (i) PCRE supports the possessive quantifier  "++"  syntax,  taken  from
       Sun's Java package.

       (j) The (R) condition, for testing recursion, is a PCRE extension.

       (k) The callout facility is PCRE-specific.

       (l) The partial matching facility is PCRE-specific.

       (m) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.

       (n) The alternative matching function (pcre_dfa_exec())  matches  in  a
       different way and is not Perl-compatible.

Last updated: 06 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREPATTERN(3)                                                  PCREPATTERN(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

       The  syntax  and semantics of the regular expressions supported by PCRE
       are described below. Regular expressions are also described in the Perl
       documentation  and  in  a  number  of books, some of which have copious
       examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
       by  O'Reilly, covers regular expressions in great detail. This descrip-
       tion of PCRE's regular expressions is intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, you must build PCRE to  include  UTF-8  support,  and  then  call
       pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
       matching is mentioned in several places below. There is also a  summary
       of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
       page.

       The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE when its main matching function, pcre_exec(), is used.
       From  release  6.0,   PCRE   offers   a   second   matching   function,
       pcre_dfa_exec(),  which matches using a different algorithm that is not
       Perl-compatible. The advantages and disadvantages  of  the  alternative
       function, and how it differs from the normal function, are discussed in
       the pcrematching page.

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE_CASELESS option), letters  are
       matched  independently  of case. In UTF-8 mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is always possible. For characters with higher val-
       ues, the concept of case is supported if PCRE is compiled with  Unicode
       property  support,  but  not  otherwise.   If  you want to use caseless
       matching for characters 128 and above, you must  ensure  that  PCRE  is
       compiled with Unicode property support as well as with UTF-8 support.

       The  power  of  regular  expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized in square brackets. Outside  square  brackets,  the
       metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The following sections describe the use of each of the  metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a non-alphanumeric character, it takes away any  special  meaning  that
       character  may  have.  This  use  of  backslash  as an escape character
       applies both inside and outside character classes.

       For example, if you want to match a * character, you write  \*  in  the
       pattern.   This  escaping  action  applies whether or not the following
       character would otherwise be interpreted as a metacharacter, so  it  is
       always  safe  to  precede  a non-alphanumeric with backslash to specify
       that it stands for itself. In particular, if you want to match a  back-
       slash, you write \\.

       If  a  pattern is compiled with the PCRE_EXTENDED option, whitespace in
       the pattern (other than in a character class) and characters between  a
       # outside a character class and the next newline are ignored. An escap-
       ing backslash can be used to include a whitespace  or  #  character  as
       part of the pattern.

       If  you  want  to remove the special meaning from a sequence of charac-
       ters, you can do so by putting them between \Q and \E. This is  differ-
       ent  from  Perl  in  that  $  and  @ are handled as literals in \Q...\E
       sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters  in patterns in a visible manner. There is no restriction on the
       appearance of non-printing characters, apart from the binary zero  that
       terminates  a  pattern,  but  when  a pattern is being prepared by text
       editing, it is usually easier  to  use  one  of  the  following  escape
       sequences than the binary character it represents:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        newline (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or backreference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh..

       The  precise  effect of \cx is as follows: if x is a lower case letter,
       it is converted to upper case. Then bit 6 of the character (hex 40)  is
       inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
       becomes hex 7B.

       After \x, from zero to two hexadecimal digits are read (letters can  be
       in  upper  or  lower case). Any number of hexadecimal digits may appear
       between \x{ and }, but the value of the character  code  must  be  less
       than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
       the maximum hexadecimal value is 7FFFFFFF). If  characters  other  than
       hexadecimal  digits  appear between \x{ and }, or if there is no termi-
       nating }, this form of escape is not recognized.  Instead, the  initial
       \x will be interpreted as a basic hexadecimal escape, with no following
       digits, giving a character whose value is zero.

       Characters whose value is less than 256 can be defined by either of the
       two  syntaxes  for  \x. There is no difference in the way they are han-
       dled. For example, \xdc is exactly the same as \x{dc}.

       After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those  that  are  present  are used. Thus the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code  value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its as a decimal number. If the number is less than  10,  or  if  there
       have been at least that many previous capturing left parentheses in the
       expression, the entire  sequence  is  taken  as  a  back  reference.  A
       description  of how this works is given later, following the discussion
       of parenthesized subpatterns.

       Inside a character class, or if the decimal number is  greater  than  9
       and  there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the backslash, ane uses them to gen-
       erate  a data character. Any subsequent digits stand for themselves. In
       non-UTF-8 mode, the value of a character specified  in  octal  must  be
       less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
       example:

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

       Note that octal values of 100 or greater must not be  introduced  by  a
       leading zero, because no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class,  the  sequence \b is interpreted as the backspace character (hex
       08), and the sequence \X is interpreted as the character "X". Outside a
       character class, these sequences have different meanings (see below).

   Generic character types

       The  third  use of backslash is for specifying generic character types.
       The following are always recognized:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \s     any whitespace character
         \S     any character that is not a whitespace character
         \w     any "word" character
         \W     any "non-word" character

       Each pair of escape sequences partitions the complete set of characters
       into  two disjoint sets. Any given character matches one, and only one,
       of each pair.

       These character type sequences can appear both inside and outside char-
       acter  classes.  They each match one character of the appropriate type.
       If the current matching point is at the end of the subject string,  all
       of them fail, since there is no character to match.

       For  compatibility  with Perl, \s does not match the VT character (code
       11).  This makes it different from the the POSIX "space" class. The  \s
       characters  are  HT (9), LF (10), FF (12), CR (13), and space (32). (If
       "use locale;" is included in a Perl script, \s may match the VT charac-
       ter. In PCRE, it never does.)

       A "word" character is an underscore or any character less than 256 that
       is a letter or digit. The definition of  letters  and  digits  is  con-
       trolled  by PCRE's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the  pcreapi
       page).  For  example,  in  the  "fr_FR" (French) locale, some character
       codes greater than 128 are used for accented  letters,  and  these  are
       matched by \w.

       In  UTF-8 mode, characters with values greater than 128 never match \d,
       \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
       code  character  property support is available. The use of locales with
       Unicode is discouraged.

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional  escape  sequences  to  match character properties are available
       when UTF-8 mode is selected. They are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

       The property names represented by xx above are limited to  the  Unicode
       script names, the general category properties, and "Any", which matches
       any character (including newline). Other properties such as "InMusical-
       Symbols"  are  not  currently supported by PCRE. Note that \P{Any} does
       not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

         \p{Greek}
         \P{Han}

       Those that are not part of an identified script are lumped together  as
       "Common". The current list of scripts is:

       Arabic,  Armenian,  Bengali,  Bopomofo, Braille, Buginese, Buhid, Cana-
       dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic,  Deseret,
       Devanagari,  Ethiopic,  Georgian,  Glagolitic, Gothic, Greek, Gujarati,
       Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,  Inherited,  Kannada,
       Katakana,  Kharoshthi,  Khmer,  Lao, Latin, Limbu, Linear_B, Malayalam,
       Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya,
       Osmanya,  Runic,  Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag-
       banwa,  Tai_Le,  Tamil,  Telugu,  Thaana,  Thai,   Tibetan,   Tifinagh,
       Ugaritic, Yi.

       Each  character has exactly one general category property, specified by
       a two-letter abbreviation. For compatibility with Perl, negation can be
       specified  by  including a circumflex between the opening brace and the
       property name. For example, \p{^Lu} is the same as \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  long  synonyms  for  these  properties that Perl supports (such as
       \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper case letters.

       The \X escape matches any number of Unicode  characters  that  form  an
       extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

       That  is,  it matches a character without the "mark" property, followed
       by zero or more characters with the "mark"  property,  and  treats  the
       sequence  as  an  atomic group (see below).  Characters with the "mark"
       property are typically accents that affect the preceding character.

       Matching characters by Unicode property is not fast, because  PCRE  has
       to  search  a  structure  that  contains data for over fifteen thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE.

   Simple assertions

       The fourth use of backslash is for certain simple assertions. An asser-
       tion specifies a condition that has to be met at a particular point  in
       a  match, without consuming any characters from the subject string. The
       use of subpatterns for more complicated assertions is described  below.
       The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at start of subject
         \Z     matches at end of subject or before newline at end
         \z     matches at end of subject
         \G     matches at first matching position in subject

       These  assertions may not appear in character classes (but note that \b
       has a different meaning, namely the backspace character, inside a char-
       acter class).

       A  word  boundary is a position in the subject string where the current
       character and the previous character do not both match \w or  \W  (i.e.
       one  matches  \w  and the other matches \W), or the start or end of the
       string if the first or last character matches \w, respectively.

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in the next section) in that they only ever match
       at the very start and end of the subject string, whatever  options  are
       set.  Thus,  they are independent of multiline mode. These three asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect  only the behaviour of the circumflex and dollar metacharacters.
       However, if the startoffset argument of pcre_exec() is non-zero,  indi-
       cating that matching is to start at a point other than the beginning of
       the subject, \A can never match. The difference between \Z  and  \z  is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

       The \G assertion is true only when the current matching position is  at
       the  start point of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times with appropriate argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note,  however,  that  PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end  of  the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one  match
       at a time, it cannot reproduce this behaviour.

       If  all  the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
       PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
       has an entirely different meaning (see below).

       Circumflex need not be the first character of the pattern if  a  number
       of  alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever  to  match  that
       branch.  If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start  of  the  sub-
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       A dollar character is an assertion that is true  only  if  the  current
       matching  point  is  at  the  end of the subject string, or immediately
       before a newline at the end of the string (by default). Dollar need not
       be  the  last  character of the pattern if a number of alternatives are
       involved, but it should be the last item in  any  branch  in  which  it
       appears. Dollar has no special meaning in a character class.

       The  meaning  of  dollar  can be changed so that it matches only at the
       very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex
       matches  immediately after internal newlines as well as at the start of
       the subject string. It does not match after a  newline  that  ends  the
       string.  A dollar matches before any newlines in the string, as well as
       at the very end, when PCRE_MULTILINE is set. When newline is  specified
       as  the  two-character  sequence CRLF, isolated CR and LF characters do
       not indicate newlines.

       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
       (where  \n  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
       PCRE_MULTILINE is set.

       Note that the sequences \A, \Z, and \z can be used to match  the  start
       and  end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not  PCRE_MULTILINE  is
       set.


FULL STOP (PERIOD, DOT)

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies  the  end  of  a line. In UTF-8 mode, the matched character may be
       more than one byte long. When a line ending  is  defined  as  a  single
       character  (CR  or LF), dot never matches that character; when the two-
       character sequence CRLF is used, dot does not match CR if it is immedi-
       ately  followed by LF, but otherwise it matches all characters (includ-
       ing isolated CRs and LFs).

       The behaviour of dot with regard to newlines can  be  changed.  If  the
       PCRE_DOTALL  option  is  set,  a dot matches any one character, without
       exception. If newline is defined as the two-character sequence CRLF, it
       takes two dots to match it.

       The  handling of dot is entirely independent of the handling of circum-
       flex and dollar, the only relationship being  that  they  both  involve
       newlines. Dot has no special meaning in a character class.


MATCHING A SINGLE BYTE

       Outside a character class, the escape sequence \C matches any one byte,
       both in and out of UTF-8 mode. Unlike a dot, it always matches  CR  and
       LF.  The feature is provided in Perl in order to match individual bytes
       in UTF-8 mode.  Because it breaks up UTF-8 characters  into  individual
       bytes,  what remains in the string may be a malformed UTF-8 string. For
       this reason, the \C escape sequence is best avoided.

       PCRE does not allow \C to appear in  lookbehind  assertions  (described
       below),  because  in UTF-8 mode this would make it impossible to calcu-
       late the length of the lookbehind.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial. If a closing square bracket is required as a member of the class,
       it  should  be  the first data character in the class (after an initial
       circumflex, if present) or escaped with a backslash.

       A character class matches a single character in the subject.  In  UTF-8
       mode,  the character may occupy more than one byte. A matched character
       must be in the set of characters defined by the class, unless the first
       character  in  the  class definition is a circumflex, in which case the
       subject character must not be in the set defined by  the  class.  If  a
       circumflex  is actually required as a member of the class, ensure it is
       not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case  vowel,
       while  [^aeiou]  matches  any character that is not a lower case vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters  that  are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion: it still  con-
       sumes  a  character  from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

       In UTF-8 mode, characters with values greater than 255 can be  included
       in  a  class as a literal string of bytes, or by using the \x{ escaping
       mechanism.

       When caseless matching is set, any letters in a  class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported  if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching for characters 128 and above, you  must  ensure  that
       PCRE  is  compiled  with Unicode property support as well as with UTF-8
       support.

       Characters that might indicate  line  breaks  (CR  and  LF)  are  never
       treated  in  any  special way when matching character classes, whatever
       line-ending sequence is in use, and whatever setting of the PCRE_DOTALL
       and PCRE_MULTILINE options is used. A class such as [^a] always matches
       one of these characters.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character  class.  For  example,  [d-m] matches any letter
       between d and m, inclusive. If a  minus  character  is  required  in  a
       class,  it  must  be  escaped  with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as  the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges operate in the collating sequence of character values. They  can
       also   be  used  for  characters  specified  numerically,  for  example
       [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
       are greater than 255, for example [\x{100}-\x{2ff}].

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
       character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches
       accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
       concept of case for characters with values greater than 128  only  when
       it is compiled with Unicode property support.

       The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
       in a character class, and add the characters that  they  match  to  the
       class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
       flex can conveniently be used with the upper case  character  types  to
       specify  a  more  restricted  set of characters than the matching lower
       case type. For example, the class [^\W_] matches any letter  or  digit,
       but not underscore.

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see  the
       next  section),  and  the  terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by  [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT  character  (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.


VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns.  For
       example, the pattern

         gilbert|sullivan

       matches  either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty  alternative  is  permitted  (matching  the  empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the  alternatives
       are  within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the  subpattern.


INTERNAL OPTION SETTING

       The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
       PCRE_EXTENDED options can be changed  from  within  the  pattern  by  a
       sequence  of  Perl  option  letters  enclosed between "(?" and ")". The
       option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
       LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
       is also permitted. If a  letter  appears  both  before  and  after  the
       hyphen, the option is unset.

       When  an option change occurs at top level (that is, not inside subpat-
       tern parentheses), the change applies to the remainder of  the  pattern
       that follows.  If the change is placed right at the start of a pattern,
       PCRE extracts it into the global options (and it will therefore show up
       in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern affects only that part of the cur-
       rent pattern that follows it, so

         (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).   By  this means, options can be made to have different settings
       in different parts of the pattern. Any changes made in one  alternative
       do  carry  on  into subsequent branches within the same subpattern. For
       example,

         (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
       first  branch  is  abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There  would  be
       some very weird behaviour otherwise.

       The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
       can be changed in the same way as the Perl-compatible options by  using
       the characters J, U and X respectively.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches one of the words "cat", "cataract", or  "caterpillar".  Without
       the  parentheses,  it  would  match "cataract", "erpillar" or the empty
       string.

       2. It sets up the subpattern as  a  capturing  subpattern.  This  means
       that,  when  the  whole  pattern  matches,  that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       ovector  argument  of pcre_exec(). Opening parentheses are counted from
       left to right (starting from 1) to obtain  numbers  for  the  capturing
       subpatterns.

       For  example,  if the string "the red king" is matched against the pat-
       tern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when a grouping subpattern is  required
       without  a capturing requirement. If an opening parenthesis is followed
       by a question mark and a colon, the subpattern does not do any  captur-
       ing,  and  is  not  counted when computing the number of any subsequent
       capturing subpatterns. For example, if the string "the white queen"  is
       matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535, and  the
       maximum  depth  of  nesting of all subpatterns, both capturing and non-
       capturing, is 200.

       As a convenient shorthand, if any option settings are required  at  the
       start  of  a  non-capturing  subpattern,  the option letters may appear
       between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried  from  left  to right, and options are not reset until the end of
       the subpattern is reached, an option setting in one branch does  affect
       subsequent  branches,  so  the above patterns match "SUNDAY" as well as
       "Saturday".


NAMED SUBPATTERNS

       Identifying capturing parentheses by number is simple, but  it  can  be
       very  hard  to keep track of the numbers in complicated regular expres-
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change.  To help with this difficulty, PCRE supports the naming of sub-
       patterns, something that Perl  does  not  provide.  The  Python  syntax
       (?P<name>...)  is  used. References to capturing parentheses from other
       parts of the pattern, such as  backreferences,  recursion,  and  condi-
       tions, can be made by name as well as by number.

       Names  consist  of  up  to  32 alphanumeric characters and underscores.
       Named capturing parentheses are still  allocated  numbers  as  well  as
       names. The PCRE API provides function calls for extracting the name-to-
       number translation table from a compiled pattern. There is also a  con-
       venience function for extracting a captured substring by name.

       By  default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time.  This  can  be useful for patterns where only one instance of the
       named parentheses can match. Suppose you want to match the  name  of  a
       weekday,  either as a 3-letter abbreviation or as the full name, and in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

         (?P<DN>Mon|Fri|Sun)(?:day)?|
         (?P<DN>Tue)(?:sday)?|
         (?P<DN>Wed)(?:nesday)?|
         (?P<DN>Thu)(?:rsday)?|
         (?P<DN>Sat)(?:urday)?

       There  are  five capturing substrings, but only one is ever set after a
       match.  The convenience  function  for  extracting  the  data  by  name
       returns  the  substring  for  the first, and in this example, the only,
       subpattern of that name that matched.  This  saves  searching  to  find
       which  numbered  subpattern  it  was. If you make a reference to a non-
       unique named subpattern from elsewhere in the  pattern,  the  one  that
       corresponds  to  the  lowest number is used. For further details of the
       interfaces for handling named subpatterns, see the  pcreapi  documenta-
       tion.


REPETITION

       Repetition  is  specified  by  quantifiers, which can follow any of the
       following items:

         a literal data character
         the . metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         an escape such as \d that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (unless it is an assertion)

       The general repetition quantifier specifies a minimum and maximum  num-
       ber  of  permitted matches, by giving the two numbers in curly brackets
       (braces), separated by a comma. The numbers must be  less  than  65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but  the  comma  is
       present,  there  is  no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of  required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches  exactly  8  digits. An opening curly bracket that appears in a
       position where a quantifier is not allowed, or one that does not  match
       the  syntax of a quantifier, is taken as a literal character. For exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended  sequences,  each of which may be several bytes long (and they
       may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present.

       For  convenience  (and  historical compatibility) the three most common
       quantifiers have single-character abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the .*  item.

       However,  if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE_UNGREEDY option is set (an option which is not available in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  to Perl's /s) is set, thus allowing the . to match newlines, the
       pattern is implicitly anchored, because whatever follows will be  tried
       against  every character position in the subject string, so there is no
       point in retrying the overall match at any position  after  the  first.
       PCRE normally treats such a pattern as though it were preceded by \A.

       In  cases  where  it  is known that the subject string contains no new-
       lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However,  there is one situation where the optimization cannot be used.
       When .*  is inside capturing parentheses that  are  the  subject  of  a
       backreference  elsewhere in the pattern, a match at the start may fail,
       and a later one succeed. Consider, for example:

         (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth  charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is  "tweedledee".  However,  if there are nested capturing subpatterns,
       the corresponding captured values may have been set in previous  itera-
       tions. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With both maximizing and minimizing repetition, failure of what follows
       normally causes the repeated item to be re-evaluated to see if  a  dif-
       ferent number of repeats allows the rest of the pattern to match. Some-
       times it is useful to prevent this, either to change the nature of  the
       match,  or  to  cause it fail earlier than it otherwise might, when the
       author of the pattern knows there is no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to  the  subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  would
       give up immediately on failing to match "foo" the first time. The nota-
       tion is a kind of special parenthesis, starting with  (?>  as  in  this
       example:

         (?>\d+)foo

       This  kind  of  parenthesis "locks up" the  part of the pattern it con-
       tains once it has matched, and a failure further into  the  pattern  is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a subpattern of  this  type  matches
       the  string  of  characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
       pared  to  adjust  the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists  of  an  additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

         \d++foo

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler forms of atomic group. However, there is no difference  in  the
       meaning  or  processing  of  a possessive quantifier and the equivalent
       atomic group.

       The possessive quantifier syntax is an extension to  the  Perl  syntax.
       Jeffrey  Friedl originated the idea (and the name) in the first edition
       of his book.  Mike McCloskey liked it, so implemented it when he  built
       Sun's Java package, and PCRE copied it from there.

       When  a  pattern  contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use  of  an
       atomic  group  is  the  only way to avoid some failing matches taking a
       very long time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE and Perl have an optimization that allows  for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens  quickly.


BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern  earlier  (that is, to its left) in the pattern, provided there
       have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it  is  always  taken  as a back reference, and causes an error only if
       there are not that many capturing left parentheses in the  entire  pat-
       tern.  In  other words, the parentheses that are referenced need not be
       to the left of the reference for numbers less than 10. A "forward  back
       reference"  of  this  type can make sense when a repetition is involved
       and the subpattern to the right has participated in an  earlier  itera-
       tion.

       It is not possible to have a numerical "forward back reference" to sub-
       pattern whose number is 10 or more. However, a back  reference  to  any
       subpattern  is  possible  using named parentheses (see below). See also
       the subsection entitled "Non-printing  characters"  above  for  further
       details of the handling of digits following a backslash.

       A  back  reference matches whatever actually matched the capturing sub-
       pattern in the current subject string, rather  than  anything  matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not  "sense and responsibility". If caseful matching is in force at the
       time of the back reference, the case of letters is relevant. For  exam-
       ple,

         ((?i)rah)\s+\1

       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
       original capturing subpattern is matched caselessly.

       Back references to named subpatterns use the Python  syntax  (?P=name).
       We could rewrite the above example as follows:

         (?P<p1>(?i)rah)\s+(?P=p1)

       A  subpattern  that  is  referenced  by  name may appear in the pattern
       before or after the reference.

       There may be more than one back reference to the same subpattern. If  a
       subpattern  has  not actually been used in a particular match, any back
       references to it always fail. For example, the pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". Because  there
       may  be  many  capturing parentheses in a pattern, all digits following
       the backslash are taken as part of a potential back  reference  number.
       If the pattern continues with a digit character, some delimiter must be
       used to terminate the back reference. If the  PCRE_EXTENDED  option  is
       set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-
       ments" below) can be used.

       A back reference that occurs inside the parentheses to which it  refers
       fails  when  the subpattern is first used, so, for example, (a\1) never
       matches.  However, such references can be useful inside  repeated  sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the subpattern,  the  back  reference  matches  the  character
       string  corresponding  to  the previous iteration. In order for this to
       work, the pattern must be such that the first iteration does  not  need
       to  match the back reference. This can be done using alternation, as in
       the example above, or by a quantifier with a minimum of zero.


ASSERTIONS

       An assertion is a test on the characters  following  or  preceding  the
       current  matching  point that does not actually consume any characters.
       The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
       described above.

       More  complicated  assertions  are  coded as subpatterns. There are two
       kinds: those that look ahead of the current  position  in  the  subject
       string,  and  those  that  look  behind  it. An assertion subpattern is
       matched in the normal way, except that it does not  cause  the  current
       matching position to be changed.

       Assertion  subpatterns  are  not  capturing subpatterns, and may not be
       repeated, because it makes no sense to assert the  same  thing  several
       times.  If  any kind of assertion contains capturing subpatterns within
       it, these are counted for the purposes of numbering the capturing  sub-
       patterns in the whole pattern.  However, substring capturing is carried
       out only for positive assertions, because it does not  make  sense  for
       negative assertions.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches a word followed by a semicolon, but does not include the  semi-
       colon in the match, and

         foo(?!bar)

       matches  any  occurrence  of  "foo" that is not followed by "bar". Note
       that the apparently similar pattern

         (?!foo)bar

       does not find an occurrence of "bar"  that  is  preceded  by  something
       other  than "foo"; it finds any occurrence of "bar" whatsoever, because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is  with  (?!)  because  an  empty  string
       always  matches, so an assertion that requires there not to be an empty
       string must always fail.

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted  such  that  all  the
       strings it matches must have a fixed length. However, if there are sev-
       eral top-level alternatives, they do not all  have  to  have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This  is  an  extension  compared  with  Perl (at least for 5.8), which
       requires all branches to match the same length of string. An  assertion
       such as

         (?<=ab(c|de))

       is  not  permitted,  because  its single top-level branch can match two
       different lengths, but it is acceptable if rewritten to  use  two  top-
       level branches:

         (?<=abc|abde)

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the  fixed  width  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the match is deemed to fail.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode)  to appear in lookbehind assertions, because it makes it impossi-
       ble to calculate the length of the lookbehind. The \X escape, which can
       match different numbers of bytes, is also not permitted.

       Atomic  groups can be used in conjunction with lookbehind assertions to
       specify efficient matching at the end of the subject string. Consider a
       simple pattern such as

         abcd$

       when  applied  to  a  long string that does not match. Because matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and  then  see  if what follows matches the rest of the pattern. If the
       pattern is specified as

         ^.*abcd$

       the initial .* matches the entire string at first, but when this  fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so  on.  Once
       again  the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

         ^(?>.*)(?<=abcd)

       or, equivalently, using the possessive quantifier syntax,

         ^.*+(?<=abcd)

       there can be no backtracking for the .* item; it  can  match  only  the
       entire  string.  The subsequent lookbehind assertion does a single test
       on the last four characters. If it fails, the match fails  immediately.
       For  long  strings, this approach makes a significant difference to the
       processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of  the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous  three
       characters  are  all  digits,  and  then there is a check that the same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded  by  six  characters,  the first of which are digits and the last
       three of which are not "999". For example, it  doesn't  match  "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This  time  the  first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches  an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It  is possible to cause the matching process to obey a subpattern con-
       ditionally or to choose between two alternative subpatterns,  depending
       on  the result of an assertion, or whether a previous capturing subpat-
       tern matched or not. The two possible forms of  conditional  subpattern
       are

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If  the  condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. If there are more  than  two  alterna-
       tives in the subpattern, a compile-time error occurs.

       There are three kinds of condition. If the text between the parentheses
       consists of a sequence of digits, or a sequence of alphanumeric charac-
       ters  and underscores, the condition is satisfied if the capturing sub-
       pattern of that number or name has previously matched. There is a  pos-
       sible  ambiguity here, because subpattern names may consist entirely of
       digits. PCRE looks first for a named subpattern; if it cannot find  one
       and  the text consists entirely of digits, it looks for a subpattern of
       that number, which must be greater than zero.  Using  subpattern  names
       that consist entirely of digits is not recommended.

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional subpattern that tests whether the first set
       of parentheses matched or not. If they did, that is, if subject started
       with an opening parenthesis, the condition is true, and so the yes-pat-
       tern  is  executed  and  a  closing parenthesis is required. Otherwise,
       since no-pattern is not present, the  subpattern  matches  nothing.  In
       other  words,  this  pattern  matches  a  sequence  of non-parentheses,
       optionally enclosed in parentheses. Rewriting it to use a named subpat-
       tern gives this:

         (?P<OPEN> \( )?    [^()]+    (?(OPEN) \) )

       If the condition is the string (R), and there is no subpattern with the
       name R, the condition is satisfied if a recursive call to  the  pattern
       or  subpattern  has  been made. At "top level", the condition is false.
       This is a PCRE extension.  Recursive patterns are described in the next
       section.

       If  the  condition  is  not  a sequence of digits or (R), it must be an
       assertion.  This may be a positive or negative lookahead or  lookbehind
       assertion.  Consider  this  pattern,  again  containing non-significant
       white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition  is  a  positive  lookahead  assertion  that  matches  an
       optional  sequence of non-letters followed by a letter. In other words,
       it tests for the presence of at least one letter in the subject.  If  a
       letter  is found, the subject is matched against the first alternative;
       otherwise it is  matched  against  the  second.  This  pattern  matches
       strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.


COMMENTS

       The sequence (?# marks the start of a comment that continues up to  the
       next  closing  parenthesis.  Nested  parentheses are not permitted. The
       characters that make up a comment play no part in the pattern  matching
       at all.

       If  the PCRE_EXTENDED option is set, an unescaped # character outside a
       character class introduces a  comment  that  continues  to  immediately
       after the next newline in the pattern.


RECURSIVE PATTERNS

       Consider  the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of  recursion,  the  best
       that  can  be  done  is  to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to  handle  an  arbitrary  nesting
       depth.  Perl  provides  a  facility  that allows regular expressions to
       recurse (amongst other things). It does this by interpolating Perl code
       in the expression at run time, and the code can refer to the expression
       itself. A Perl pattern to solve the parentheses problem can be  created
       like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears. Obviously,  PCRE
       cannot  support  the  interpolation  of Perl code. Instead, it supports
       some special syntax for recursion of the entire pattern, and  also  for
       individual subpattern recursion.

       The  special item that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive call of the subpattern of
       the  given  number, provided that it occurs inside that subpattern. (If
       not, it is a "subroutine" call, which is described  in  the  next  sec-
       tion.)  The special item (?R) is a recursive call of the entire regular
       expression.

       A recursive subpattern call is always treated as an atomic group.  That
       is,  once  it  has  matched some of the subject string, it is never re-
       entered, even if it contains untried alternatives and there is a subse-
       quent matching failure.

       This  PCRE  pattern  solves  the nested parentheses problem (assume the
       PCRE_EXTENDED option is set so that white space is ignored):

         \( ( (?>[^()]+) | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a  correctly  parenthe-
       sized substring).  Finally there is a closing parenthesis.

       If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

         ( \( ( (?>[^()]+) | (?1) )* \) )

       We have put the pattern into parentheses, and caused the  recursion  to
       refer  to them instead of the whole pattern. In a larger pattern, keep-
       ing track of parenthesis numbers can be tricky. It may be  more  conve-
       nient  to use named parentheses instead. For this, PCRE uses (?P>name),
       which is an extension to the Python syntax that  PCRE  uses  for  named
       parentheses (Perl does not provide named parentheses). We could rewrite
       the above example as follows:

         (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )

       This particular example pattern contains nested unlimited repeats,  and
       so  the  use of atomic grouping for matching strings of non-parentheses
       is important when applying the pattern to strings that  do  not  match.
       For example, when this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields "no match" quickly. However, if atomic grouping is not used,
       the match runs for a very long time indeed because there  are  so  many
       different  ways  the  + and * repeats can carve up the subject, and all
       have to be tested before failure can be reported.

       At the end of a match, the values set for any capturing subpatterns are
       those from the outermost level of the recursion at which the subpattern
       value is set.  If you want to obtain  intermediate  values,  a  callout
       function can be used (see the next section and the pcrecallout documen-
       tation). If the pattern above is matched against

         (ab(cd)ef)

       the value for the capturing parentheses is  "ef",  which  is  the  last
       value  taken  on at the top level. If additional parentheses are added,
       giving

         \( ( ( (?>[^()]+) | (?R) )* ) \)
            ^                        ^
            ^                        ^

       the string they capture is "ab(cd)ef", the contents of  the  top  level
       parentheses.  If there are more than 15 capturing parentheses in a pat-
       tern, PCRE has to obtain extra memory to store data during a recursion,
       which  it  does  by  using pcre_malloc, freeing it via pcre_free after-
       wards. If  no  memory  can  be  obtained,  the  match  fails  with  the
       PCRE_ERROR_NOMEMORY error.

       Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In  this  pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and  non-recursive  cases.
       The (?R) item is the actual recursive call.


SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern reference (either by number or
       by name) is used outside the parentheses to which it refers,  it  oper-
       ates  like  a  subroutine in a programming language. An earlier example
       pointed out that the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Such references, if given  numerically,  must  follow  the
       subpattern  to which they refer. However, named references can refer to
       later subpatterns.

       Like recursive subpatterns, a "subroutine" call is always treated as an
       atomic  group. That is, once it has matched some of the subject string,
       it is never re-entered, even if it contains  untried  alternatives  and
       there is a subsequent matching failure.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout  points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the  number
       of  the callout, the position in the pattern, and, optionally, one item
       of data originally supplied by the caller of pcre_exec().  The  callout
       function  may cause matching to proceed, to backtrack, or to fail alto-
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.

Last updated: 06 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREPARTIAL(3)                                                  PCREPARTIAL(3)


NAME
       PCRE - Perl-compatible regular expressions


PARTIAL MATCHING IN PCRE

       In  normal  use  of  PCRE,  if  the  subject  string  that is passed to
       pcre_exec() or pcre_dfa_exec() matches as far as it goes,  but  is  too
       short  to  match  the  entire  pattern, PCRE_ERROR_NOMATCH is returned.
       There are circumstances where it might be helpful to  distinguish  this
       case from other cases in which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error as soon as a mistake is made, possibly beeping and not
       reflecting the character that has been typed. This  immediate  feedback
       is  likely  to  be a better user interface than a check that is delayed
       until the entire string has been entered.

       PCRE supports the concept of partial matching by means of the PCRE_PAR-
       TIAL   option,   which   can   be   set  when  calling  pcre_exec()  or
       pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
       PCRE_ERROR_NOMATCH  is converted into PCRE_ERROR_PARTIAL if at any time
       during the matching process the last part of the subject string matched
       part  of  the  pattern. Unfortunately, for non-anchored matching, it is
       not possible to obtain the position of the start of the partial  match.
       No captured data is set when PCRE_ERROR_PARTIAL is returned.

       When   PCRE_PARTIAL   is  set  for  pcre_dfa_exec(),  the  return  code
       PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the  end  of
       the  subject is reached, there have been no complete matches, but there
       is still at least one matching possibility. The portion of  the  string
       that provided the partial match is set as the first matching string.

       Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
       the last literal byte in a pattern, and abandons  matching  immediately
       if  such a byte is not present in the subject string. This optimization
       cannot be used for a subject string that might match only partially.


RESTRICTED PATTERNS FOR PCRE_PARTIAL

       Because of the way certain internal optimizations  are  implemented  in
       the  pcre_exec()  function, the PCRE_PARTIAL option cannot be used with
       all patterns. These restrictions do not apply when  pcre_dfa_exec()  is
       used.  For pcre_exec(), repeated single characters such as

         a{2,4}

       and repeated single metasequences such as

         \d+

       are  not permitted if the maximum number of occurrences is greater than
       one.  Optional items such as \d? (where the maximum is one) are permit-
       ted.   Quantifiers  with any values are permitted after parentheses, so
       the invalid examples above can be coded thus:

         (a){2,4}
         (\d)+

       These constructions run more slowly, but for the kinds  of  application
       that  are  envisaged  for this facility, this is not felt to be a major
       restriction.

       If PCRE_PARTIAL is set for a pattern  that  does  not  conform  to  the
       restrictions,  pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
       (-13).


EXAMPLE OF PARTIAL MATCHING USING PCRETEST

       If the escape sequence \P is present  in  a  pcretest  data  line,  the
       PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
       uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P
          0: 25jun04
          1: jun
         data> 25dec3\P
         Partial match
         data> 3ju\P
         Partial match
         data> 3juj\P
         No match
         data> j\P
         No match

       The first data string is matched  completely,  so  pcretest  shows  the
       matched  substrings.  The  remaining four strings do not match the com-
       plete pattern, but the first two are partial matches.  The  same  test,
       using  DFA  matching (by means of the \D escape sequence), produces the
       following output:

           re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/
         data> 25jun04\P\D
          0: 25jun04
         data> 23dec3\P\D
         Partial match: 23dec3
         data> 3ju\P\D
         Partial match: 3ju
         data> 3juj\P\D
         No match
         data> j\P\D
         No match

       Notice that in this case the portion of the string that was matched  is
       made available.


MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()

       When a partial match has been found using pcre_dfa_exec(), it is possi-
       ble to continue the match by  providing  additional  subject  data  and
       calling  pcre_dfa_exec() again with the PCRE_DFA_RESTART option and the
       same working space (where details of the  previous  partial  match  are
       stored).  Here  is  an  example  using  pcretest,  where  the \R escape
       sequence sets the PCRE_DFA_RESTART option and the  \D  escape  sequence
       requests the use of pcre_dfa_exec():

           re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/
         data> 23ja\P\D
         Partial match: 23ja
         data> n05\R\D
          0: n05

       The  first  call has "23ja" as the subject, and requests partial match-
       ing; the second call  has  "n05"  as  the  subject  for  the  continued
       (restarted)  match.   Notice  that when the match is complete, only the
       last part is shown; PCRE does  not  retain  the  previously  partially-
       matched  string. It is up to the calling program to do that if it needs
       to.

       This facility can  be  used  to  pass  very  long  subject  strings  to
       pcre_dfa_exec(). However, some care is needed for certain types of pat-
       tern.

       1. If the pattern contains tests for the beginning or end  of  a  line,
       you  need  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
       ate, when the subject string for any call does not contain  the  begin-
       ning or end of a line.

       2.  If  the  pattern contains backward assertions (including \b or \B),
       you need to arrange for some overlap in the subject  strings  to  allow
       for  this.  For example, you could pass the subject in chunks that were
       500 bytes long, but in a buffer of 700 bytes, with the starting  offset
       set to 200 and the previous 200 bytes at the start of the buffer.

       3.  Matching a subject string that is split into multiple segments does
       not always produce exactly the same result as matching over one  single
       long  string.   The  difference arises when there are multiple matching
       possibilities, because a partial match result is given only when  there
       are  no  completed  matches  in a call to fBpcre_dfa_exec(). This means
       that as soon as the shortest match has been found,  continuation  to  a
       new  subject  segment  is  no  longer possible.  Consider this pcretest
       example:

           re> /dog(sbody)?/
         data> do\P\D
         Partial match: do
         data> gsb\R\P\D
          0: g
         data> dogsbody\D
          0: dogsbody
          1: dog

       The pattern matches the words "dog" or "dogsbody". When the subject  is
       presented  in  several  parts  ("do" and "gsb" being the first two) the
       match stops when "dog" has been found, and it is not possible  to  con-
       tinue.  On  the  other  hand,  if  "dogsbody"  is presented as a single
       string, both matches are found.

       Because of this phenomenon, it does not usually make  sense  to  end  a
       pattern that is going to be matched in this way with a variable repeat.

       4. Patterns that contain alternatives at the top level which do not all
       start with the same pattern item may not work as expected. For example,
       consider this pattern:

         1234|3789

       If the first part of the subject is "ABC123", a partial  match  of  the
       first  alternative  is found at offset 3. There is no partial match for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "789" does not yield a match because only those alternatives that match
       at  one point in the subject are remembered. The problem arises because
       the start of the second alternative matches within the  first  alterna-
       tive. There is no problem with anchored patterns or patterns such as:

         1234|ABCD

       where no string can be a partial match for both alternatives.

Last updated: 16 January 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREPRECOMPILE(3)                                            PCREPRECOMPILE(3)


NAME
       PCRE - Perl-compatible regular expressions


SAVING AND RE-USING PRECOMPILED PCRE PATTERNS

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit more complicated.

       If you save compiled patterns to a file, you can copy them to a differ-
       ent  host  and  run them there. This works even if the new host has the
       opposite endianness to the one on which  the  patterns  were  compiled.
       There  may  be a small performance penalty, but it should be insignifi-
       cant.


SAVING A COMPILED PATTERN
       The value returned by pcre_compile() points to a single block of memory
       that  holds  the compiled pattern and associated data. You can find the
       length of this block in bytes by calling pcre_fullinfo() with an  argu-
       ment  of  PCRE_INFO_SIZE. You can then save the data in any appropriate
       manner. Here is sample code that compiles a pattern and writes it to  a
       file. It assumes that the variable fd refers to a file that is open for
       output:

         int erroroffset, rc, size;
         char *error;
         pcre *re;

         re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

       In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

       Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

       If  the pattern has been studied, it is also possible to save the study
       data in a similar way to the compiled  pattern  itself.  When  studying
       generates  additional  information, pcre_study() returns a pointer to a
       pcre_extra data block. Its format is defined in the section on matching
       a  pattern in the pcreapi documentation. The study_data field points to
       the binary study data,  and  this  is  what  you  must  save  (not  the
       pcre_extra  block itself). The length of the study data can be obtained
       by calling pcre_fullinfo() with  an  argument  of  PCRE_INFO_STUDYSIZE.
       Remember  to check that pcre_study() did return a non-NULL value before
       trying to save the study data.


RE-USING A PRECOMPILED PATTERN

       Re-using a precompiled pattern is straightforward. Having  reloaded  it
       into   main   memory,   you   pass   its   pointer  to  pcre_exec()  or
       pcre_dfa_exec() in the usual way. This  should  work  even  on  another
       host,  and  even  if  that  host has the opposite endianness to the one
       where the pattern was compiled.

       However, if you passed a pointer to custom character  tables  when  the
       pattern  was  compiled  (the  tableptr argument of pcre_compile()), you
       must now pass a similar  pointer  to  pcre_exec()  or  pcre_dfa_exec(),
       because  the  value  saved  with the compiled pattern will obviously be
       nonsense. A field in a pcre_extra() block is used to pass this data, as
       described  in the section on matching a pattern in the pcreapi documen-
       tation.

       If you did not provide custom character tables  when  the  pattern  was
       compiled,  the  pointer  in  the compiled pattern is NULL, which causes
       pcre_exec() to use PCRE's internal tables. Thus, you  do  not  need  to
       take any special action at run time in this case.

       If  you  saved study data with the compiled pattern, you need to create
       your own pcre_extra data block and set the study_data field to point to
       the  reloaded  study  data. You must also set the PCRE_EXTRA_STUDY_DATA
       bit in the flags field to indicate that study  data  is  present.  Then
       pass  the  pcre_extra  block  to  pcre_exec() or pcre_dfa_exec() in the
       usual way.


COMPATIBILITY WITH DIFFERENT PCRE RELEASES

       The layout of the control block that is at the start of the  data  that
       makes  up  a  compiled pattern was changed for release 5.0. If you have
       any saved patterns that were compiled with  previous  releases  (not  a
       facility  that  was  previously advertised), you will have to recompile
       them for release 5.0. However, from now on, it should  be  possible  to
       make changes in a compatible manner.

       Notwithstanding the above, if you have any saved patterns in UTF-8 mode
       that use \p or \P that were compiled with any release up to and includ-
       ing 6.4, you will have to recompile them for release 6.5 and above.

Last updated: 01 February 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCREPERFORM(3)                                                  PCREPERFORM(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE PERFORMANCE

       Certain  items  that may appear in regular expression patterns are more
       efficient than others. It is more efficient to use  a  character  class
       like  [aeiou]  than  a set of alternatives such as (a|e|i|o|u). In gen-
       eral, the simplest construction that provides the required behaviour is
       usually  the  most  efficient.  Jeffrey Friedl's book contains a lot of
       useful general discussion  about  optimizing  regular  expressions  for
       efficient  performance. This document contains a few observations about
       PCRE.

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a  character's  property.
       If  you  can  find  an  alternative pattern that does not use character
       properties, it will probably be faster.

       When a pattern begins with .* not in  parentheses,  or  in  parentheses
       that are not the subject of a backreference, and the PCRE_DOTALL option
       is set, the pattern is implicitly anchored by PCRE, since it can  match
       only  at  the start of a subject string. However, if PCRE_DOTALL is not
       set, PCRE cannot make this optimization, because  the  .  metacharacter
       does  not then match a newline, and if the subject string contains new-
       lines, the pattern may match from the character  immediately  following
       one of them instead of from the very start. For example, the pattern

         .*second

       matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to do this, PCRE has to retry the match starting after every newline in
       the subject.

       If you are using such a pattern with subject strings that do  not  con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or starting the pattern with ^.* or ^.*? to indicate  explicit  anchor-
       ing.  That saves PCRE from having to scan along the subject looking for
       a newline to restart at.

       Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

         (a+)*

       This can match "aaaa" in 33 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those  cases  other  than  0,  the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail, PCRE has in
       principle  to  try  every  possible  variation,  and  this  can take an
       extremely long time.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before  embarking  on  the  standard
       matching  procedure,  PCRE checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.

Last updated: 28 February 2005
Copyright (c) 1997-2005 University of Cambridge.
------------------------------------------------------------------------------


PCREPOSIX(3)                                                      PCREPOSIX(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF POSIX API

       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION

       This  set  of  functions provides a POSIX-style API to the PCRE regular
       expression package. See the pcreapi documentation for a description  of
       PCRE's native API, which contains much additional functionality.

       The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h  header  file,  and  on  Unix systems the library itself is
       called pcreposix.a, so can be accessed by  adding  -lpcreposix  to  the
       command  for  linking  an application that uses them. Because the POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I have implemented only those option bits that can be reasonably mapped
       to PCRE native options. In addition, the option REG_EXTENDED is defined
       with the value zero. This has no effect, but since  programs  that  are
       written  to  the  POSIX interface often use it, this makes it easier to
       slot in PCRE as a replacement library. Other POSIX options are not even
       defined.

       When  PCRE  is  called  via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE options, as described below. "POSIX-like in  style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-byte encoding  domains  it  is  probably
       even less compatible.

       The  header for these functions is supplied as pcreposix.h to avoid any
       potential clash with other POSIX  libraries.  It  can,  of  course,  be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two structure types, regex_t for  compiled  internal  forms,  and  reg-
       match_t  for  returning  captured substrings. It also defines some con-
       stants whose names start  with  "REG_";  these  are  used  for  setting
       options and identifying error codes.


COMPILING A PATTERN

       The  function regcomp() is called to compile a pattern into an internal
       form. The pattern is a C string terminated by a  binary  zero,  and  is
       passed  in  the  argument  pattern. The preg argument is a pointer to a
       regex_t structure that is used as a base for storing information  about
       the compiled regular expression.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The PCRE_DOTALL option is set when the regular expression is passed for
       compilation to the native function. Note that REG_DOTALL is not part of
       the POSIX standard.

         REG_ICASE

       The PCRE_CASELESS option is set when the regular expression  is  passed
       for compilation to the native function.

         REG_NEWLINE

       The  PCRE_MULTILINE option is set when the regular expression is passed
       for compilation to the native function. Note that this does  not  mimic
       the  defined  POSIX  behaviour  for REG_NEWLINE (see the following sec-
       tion).

         REG_NOSUB

       The PCRE_NO_AUTO_CAPTURE option is set when the regular  expression  is
       passed for compilation to the native function. In addition, when a pat-
       tern that is compiled with this flag is passed to regexec() for  match-
       ing,  the  nmatch  and  pmatch  arguments  are ignored, and no captured
       strings are returned.

         REG_UTF8

       The PCRE_UTF8 option is set when the regular expression is  passed  for
       compilation  to the native function. This causes the pattern itself and
       all data strings used for matching it to be treated as  UTF-8  strings.
       Note that REG_UTF8 is not part of the POSIX standard.

       In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the  regex  is  compiled  with  PCRE  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they  aren't)  or
       by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE to obey  POSIX  semantics,  but
       then  PCRE was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching  newline  characters  in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl,  there  is
       no way to stop newline from matching [^a].

       The   default  POSIX  newline  handling  can  be  obtained  by  setting
       PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to  make  PCRE
       behave exactly as for the REG_NEWLINE action.


MATCHING A PATTERN

       The  function  regexec()  is  called  to  match a compiled pattern preg
       against a given string, which is terminated by a zero byte, subject  to
       the options in eflags. These can be:

         REG_NOTBOL

       The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEOL

       The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       regexec() are ignored.

       Otherwise,the portion of the string that was matched, and also any cap-
       tured substrings, are returned via the pmatch argument, which points to
       an  array  of nmatch structures of type regmatch_t, containing the mem-
       bers rm_so and rm_eo. These contain the offset to the  first  character
       of  each  substring and the offset to the first character after the end
       of each substring, respectively. The 0th element of the vector  relates
       to  the  entire portion of string that was matched; subsequent elements
       relate to the capturing subpatterns of the regular  expression.  Unused
       entries in the array have both structure members set to -1.

       A  successful  match  yields  a  zero  return;  various error codes are
       defined in the header file, of  which  REG_NOMATCH  is  the  "expected"
       failure code.


ERROR MESSAGES

       The regerror() function maps a non-zero errorcode from either regcomp()
       or regexec() to a printable message. If preg is  not  NULL,  the  error
       should have arisen from the use of that structure. A message terminated
       by a binary zero is placed  in  errbuf.  The  length  of  the  message,
       including  the  zero, is limited to errbuf_size. The yield of the func-
       tion is the size of buffer needed to hold the whole message.


MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with  the preg structure. The function regfree() frees all such
       memory, after which preg may no longer be used as  a  compiled  expres-
       sion.


AUTHOR

       Philip Hazel
       University Computing Service,
       Cambridge CB2 3QG, England.

Last updated: 16 January 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


PCRECPP(3)                                                          PCRECPP(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF C++ WRAPPER

       #include <pcrecpp.h>


DESCRIPTION

       The  C++  wrapper  for PCRE was provided by Google Inc. Some additional
       functionality was added by Giuseppe Maxia. This brief man page was con-
       structed  from  the  notes  in the pcrecpp.h file, which should be con-
       sulted for further details.


MATCHING INTERFACE

       The "FullMatch" operation checks that supplied text matches a  supplied
       pattern  exactly.  If pointer arguments are supplied, it copies matched
       sub-strings that match sub-patterns into them.

         Example: successful match
            pcrecpp::RE re("h.*o");
            re.FullMatch("hello");

         Example: unsuccessful match (requires full match):
            pcrecpp::RE re("e");
            !re.FullMatch("hello");

         Example: creating a temporary RE object:
            pcrecpp::RE("h.*o").FullMatch("hello");

       You can pass in a "const char*" or a "string" for "text". The  examples
       below  tend to use a const char*. You can, as in the different examples
       above, store the RE object explicitly in a variable or use a  temporary
       RE  object.  The  examples below use one mode or the other arbitrarily.
       Either could correctly be used for any of these examples.

       You must supply extra pointer arguments to extract matched subpieces.

         Example: extracts "ruby" into "s" and 1234 into "i"
            int i;
            string s;
            pcrecpp::RE re("(\\w+):(\\d+)");
            re.FullMatch("ruby:1234", &s, &i);

         Example: does not try to extract any extra sub-patterns
            re.FullMatch("ruby:1234", &s);

         Example: does not try to extract into NULL
            re.FullMatch("ruby:1234", NULL, &i);

         Example: integer overflow causes failure
            !re.FullMatch("ruby:1234567891234", NULL, &i);

         Example: fails because there aren't enough sub-patterns:
            !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);

         Example: fails because string cannot be stored in integer
            !pcrecpp::RE("(.*)").FullMatch("ruby", &i);

       The provided pointer arguments can be pointers to  any  scalar  numeric
       type, or one of:

          string        (matched piece is copied to string)
          StringPiece   (StringPiece is mutated to point to matched piece)
          T             (where "bool T::ParseFrom(const char*, int)" exists)
          NULL          (the corresponding matched sub-pattern is not copied)

       The  function returns true iff all of the following conditions are sat-
       isfied:

         a. "text" matches "pattern" exactly;

         b. The number of matched sub-patterns is >= number of supplied
            pointers;

         c. The "i"th argument has a suitable type for holding the
            string captured as the "i"th sub-pattern. If you pass in
            NULL for the "i"th argument, or pass fewer arguments than
            number of sub-patterns, "i"th captured sub-pattern is
            ignored.

       The matching interface supports at most 16 arguments per call.  If  you
       need    more,    consider    using    the    more   general   interface
       pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.


PARTIAL MATCHES

       You can use the "PartialMatch" operation when you want the  pattern  to
       match any substring of the text.

         Example: simple search for a string:
            pcrecpp::RE("ell").PartialMatch("hello");

         Example: find first number in a string:
            int number;
            pcrecpp::RE re("(\\d+)");
            re.PartialMatch("x*100 + 20", &number);
            assert(number == 100);


UTF-8 AND THE MATCHING INTERFACE

       By  default,  pattern  and text are plain text, one byte per character.
       The UTF8 flag, passed to  the  constructor,  causes  both  pattern  and
       string to be treated as UTF-8 text, still a byte stream but potentially
       multiple bytes per character. In practice, the text is likelier  to  be
       UTF-8  than  the pattern, but the match returned may depend on the UTF8
       flag, so always use it when matching UTF8 text. For example,  "."  will
       match  one  byte normally but with UTF8 set may match up to three bytes
       of a multi-byte character.

         Example:
            pcrecpp::RE_Options options;
            options.set_utf8();
            pcrecpp::RE re(utf8_pattern, options);
            re.FullMatch(utf8_string);

         Example: using the convenience function UTF8():
            pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
            re.FullMatch(utf8_string);

       NOTE: The UTF8 flag is ignored if pcre was not configured with the
             --enable-utf8 flag.


PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE

       PCRE defines some modifiers to  change  the  behavior  of  the  regular
       expression   engine.  The  C++  wrapper  defines  an  auxiliary  class,
       RE_Options, as a vehicle to pass such modifiers to  a  RE  class.  Cur-
       rently, the following modifiers are supported:

          modifier              description               Perl corresponding

          PCRE_CASELESS         case insensitive match      /i
          PCRE_MULTILINE        multiple lines match        /m
          PCRE_DOTALL           dot matches newlines        /s
          PCRE_DOLLAR_ENDONLY   $ matches only at end       N/A
          PCRE_EXTRA            strict escape parsing       N/A
          PCRE_EXTENDED         ignore whitespaces          /x
          PCRE_UTF8             handles UTF8 chars          built-in
          PCRE_UNGREEDY         reverses * and *?           N/A
          PCRE_NO_AUTO_CAPTURE  disables capturing parens   N/A (*)

       (*)  Both Perl and PCRE allow non capturing parentheses by means of the
       "?:" modifier within the pattern itself. e.g. (?:ab|cd) does  not  cap-
       ture, while (ab|cd) does.

       For  a  full  account on how each modifier works, please check the PCRE
       API reference page.

       For each modifier, there are two member functions whose  name  is  made
       out  of  the  modifier  in  lowercase,  without the "PCRE_" prefix. For
       instance, PCRE_CASELESS is handled by

         bool caseless()

       which returns true if the modifier is set, and

         RE_Options & set_caseless(bool)

       which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
       be  accessed  through  the  set_match_limit()  and match_limit() member
       functions. Setting match_limit to a non-zero value will limit the  exe-
       cution  of pcre to keep it from doing bad things like blowing the stack
       or taking an eternity to return a result.  A  value  of  5000  is  good
       enough  to stop stack blowup in a 2MB thread stack. Setting match_limit
       to  zero  disables  match  limiting.  Alternatively,   you   can   call
       match_limit_recursion()  which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
       limit how much  PCRE  recurses.  match_limit()  limits  the  number  of
       matches PCRE does; match_limit_recursion() limits the depth of internal
       recursion, and therefore the amount of stack that is used.

       Normally, to pass one or more modifiers to a RE class,  you  declare  a
       RE_Options object, set the appropriate options, and pass this object to
       a RE constructor. Example:

          RE_options opt;
          opt.set_caseless(true);
          if (RE("HELLO", opt).PartialMatch("hello world")) ...

       RE_options has two constructors. The default constructor takes no argu-
       ments  and creates a set of flags that are off by default. The optional
       parameter option_flags is to facilitate transfer of legacy code from  C
       programs.  This lets you do

          RE(pattern,
            RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);

       However, new code is better off doing

          RE(pattern,
            RE_Options().set_caseless(true).set_multiline(true))
              .PartialMatch(str);

       If you are going to pass one of the most used modifiers, there are some
       convenience functions that return a RE_Options class with the appropri-
       ate  modifier  already  set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
       and EXTENDED().

       If you need to set several options at once, and you don't  want  to  go
       through  the pains of declaring a RE_Options object and setting several
       options, there is a parallel method that give you such ability  on  the
       fly.  You  can  concatenate several set_xxxxx() member functions, since
       each of them returns a reference to its class object. For  example,  to
       pass  PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
       statement, you may write:

          RE(" ^ xyz \\s+ .* blah$",
            RE_Options()
              .set_caseless(true)
              .set_extended(true)
              .set_multiline(true)).PartialMatch(sometext);


SCANNING TEXT INCREMENTALLY

       The "Consume" operation may be useful if you want to  repeatedly  match
       regular expressions at the front of a string and skip over them as they
       match. This requires use of the "StringPiece" type, which represents  a
       sub-range  of  a  real  string.  Like RE, StringPiece is defined in the
       pcrecpp namespace.

         Example: read lines of the form "var = value" from a string.
            string contents = ...;                 // Fill string somehow
            pcrecpp::StringPiece input(contents);  // Wrap in a StringPiece

            string var;
            int value;
            pcrecpp::RE re("(\\w+) = (\\d+)\n");
            while (re.Consume(&input, &var, &value)) {
              ...;
            }

       Each successful call  to  "Consume"  will  set  "var/value",  and  also
       advance "input" so it points past the matched text.

       The  "FindAndConsume"  operation  is  similar to "Consume" but does not
       anchor your match at the beginning of  the  string.  For  example,  you
       could extract all words from a string by repeatedly calling

         pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)


PARSING HEX/OCTAL/C-RADIX NUMBERS

       By default, if you pass a pointer to a numeric value, the corresponding
       text is interpreted as a base-10  number.  You  can  instead  wrap  the
       pointer with a call to one of the operators Hex(), Octal(), or CRadix()
       to interpret the text in another base. The CRadix  operator  interprets
       C-style  "0"  (base-8)  and  "0x"  (base-16)  prefixes, but defaults to
       base-10.

         Example:
           int a, b, c, d;
           pcrecpp::RE re("(.*) (.*) (.*) (.*)");
           re.FullMatch("100 40 0100 0x40",
                        pcrecpp::Octal(&a), pcrecpp::Hex(&b),
                        pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));

       will leave 64 in a, b, c, and d.


REPLACING PARTS OF STRINGS

       You can replace the first match of "pattern" in "str"  with  "rewrite".
       Within  "rewrite",  backslash-escaped  digits (\1 to \9) can be used to
       insert text matching corresponding parenthesized group  from  the  pat-
       tern. \0 in "rewrite" refers to the entire matching text. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").Replace("d", &s);

       will  leave  "s" containing "yada dabba doo". The result is true if the
       pattern matches and a replacement occurs, false otherwise.

       GlobalReplace is like Replace except that it replaces  all  occurrences
       of  the  pattern  in  the string with the rewrite. Replacements are not
       subject to re-matching. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").GlobalReplace("d", &s);

       will leave "s" containing "yada dada doo". It  returns  the  number  of
       replacements made.

       Extract  is like Replace, except that if the pattern matches, "rewrite"
       is copied into "out" (an additional argument) with substitutions.   The
       non-matching  portions  of "text" are ignored. Returns true iff a match
       occurred and the extraction happened successfully;  if no match occurs,
       the string is left unaffected.


AUTHOR

       The C++ wrapper was contributed by Google Inc.
       Copyright (c) 2005 Google Inc.
------------------------------------------------------------------------------


PCRESAMPLE(3)                                                    PCRESAMPLE(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE SAMPLE PROGRAM

       A simple, complete demonstration program, to get you started with using
       PCRE, is supplied in the file pcredemo.c in the PCRE distribution.

       The program compiles the regular expression that is its first argument,
       and  matches  it  against the subject string in its second argument. No
       PCRE options are set, and default character tables are used. If  match-
       ing  succeeds,  the  program  outputs  the  portion of the subject that
       matched, together with the contents of any captured substrings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       If PCRE is installed in the standard include  and  library  directories
       for  your  system, you should be able to compile the demonstration pro-
       gram using this command:

         gcc -o pcredemo pcredemo.c -lpcre

       If PCRE is installed elsewhere, you may need to add additional  options
       to  the  command line. For example, on a Unix-like system that has PCRE
       installed in /usr/local, you  can  compile  the  demonstration  program
       using a command like this:

         gcc -o pcredemo -I/usr/local/include pcredemo.c \
             -L/usr/local/lib -lpcre

       Once  you  have  compiled the demonstration program, you can run simple
       tests like this:

         ./pcredemo 'cat|dog' 'the cat sat on the mat'
         ./pcredemo -g 'cat|dog' 'the dog sat on the cat'

       Note that there is a  much  more  comprehensive  test  program,  called
       pcretest,  which  supports  many  more  facilities  for testing regular
       expressions and the PCRE library. The pcredemo program is provided as a
       simple coding example.

       On some operating systems (e.g. Solaris), when PCRE is not installed in
       the standard library directory, you may get an error like this when you
       try to run pcredemo:

         ld.so.1:  a.out:  fatal:  libpcre.so.0:  open failed: No such file or
       directory

       This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
------------------------------------------------------------------------------
PCRESTACK(3)                                                      PCRESTACK(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE DISCUSSION OF STACK USAGE

       When  you call pcre_exec(), it makes use of an internal function called
       match(). This calls itself recursively at branch points in the pattern,
       in  order to remember the state of the match so that it can back up and
       try a different alternative if the first one fails.  As  matching  pro-
       ceeds  deeper  and deeper into the tree of possibilities, the recursion
       depth increases.

       Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       The pcre_dfa_exec() function operates in an entirely different way, and
       hardly uses recursion at all. The limit on its complexity is the amount
       of  workspace  it  is  given.  The comments that follow do NOT apply to
       pcre_dfa_exec(); they are relevant only for pcre_exec().

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively. If the limit is exceeded, an error occurs.
       For details, see the section on  extra  data  for  pcre_exec()  in  the
       pcreapi documentation.

       Each  time  that match() is actually called recursively, it uses memory
       from the process stack. For certain kinds of  pattern  and  data,  very
       large  amounts of stack may be needed, despite the recognition of "tail
       recursion".  You can often reduce the amount of recursion,  and  there-
       fore  the  amount of stack used, by modifying the pattern that is being
       matched. Consider, for example, this pattern:

         ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet"  or  the
       end  of  the  data,  and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a  parenthesis  is  processed,  a  recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter. For a long string, a lot of stack is required. Consider  now  this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))

       This  uses very much less stack, because runs of characters that do not
       contain "<" are "swallowed" in one item inside the parentheses.  Recur-
       sion  happens  only when a "<" character that is not followed by "inet"
       is encountered (and we assume this is relatively  rare).  A  possessive
       quantifier  is  used  to stop any backtracking into the runs of non-"<"
       characters, but that is not related to stack usage.

       In environments where stack memory is constrained, you  might  want  to
       compile  PCRE to use heap memory instead of stack for remembering back-
       up points. This makes it run a lot more slowly, however. Details of how
       to do this are given in the pcrebuild documentation.

       In Unix-like environments, there is not often a problem with the stack,
       though the default limit on stack size varies from  system  to  system.
       Values  from 8Mb to 64Mb are common. You can find your default limit by
       running the command:

         ulimit -s

       The effect of running out of stack is often SIGSEGV,  though  sometimes
       an error message is given. You can normally increase the limit on stack
       size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This reads the current limits (soft and hard) using  getrlimit(),  then
       attempts  to  increase  the  soft limit to 100Mb using setrlimit(). You
       must do this before calling pcre_exec().

       PCRE has an internal counter that can be used to  limit  the  depth  of
       recursion,  and  thus cause pcre_exec() to give an error code before it
       runs out of stack. By default, the limit is very  large,  and  unlikely
       ever  to operate. It can be changed when PCRE is built, and it can also
       be set when pcre_exec() is called. For details of these interfaces, see
       the pcrebuild and pcreapi documentation.

       As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you
       should  set  the  limit at 16000 recursions. A 64Mb stack, on the other
       hand, can support around 128000 recursions. The pcretest  test  program
       has  a command line option (-S) that can be used to increase its stack.

Last updated: 29 June 2006
Copyright (c) 1997-2006 University of Cambridge.
------------------------------------------------------------------------------


