trunk/doc/pcre.txt

-----------------------------------------------------------------------------
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. Certain features that appeared in Python and
       PCRE before they appeared in Perl are also available using  the  Python
       syntax.  There is also some support for certain .NET and Oniguruma syn-
       tax items, and there is an option for  requesting  some  minor  changes
       that give better JavaScript compatibility.

       The  current  implementation of PCRE (release 7.x) corresponds approxi-
       mately with Perl 5.10, including support for UTF-8 encoded strings  and
       Unicode general category properties. However, UTF-8 and Unicode support
       has to be explicitly enabled; it is not the default. The Unicode tables
       correspond to Unicode release 5.0.0.

       In  addition to the Perl-compatible matching function, PCRE contains 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. There is a syntax summary in the  pcresyntax
       page.

       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
         pcre-config       show PCRE installation configuration information
         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
         pcresyntax        quick syntax reference
         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 is slower.

       All values in repeating quantifiers must be less than 65536.

       There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns.

       The maximum length of name for a named subpattern is 32 characters, and
       the maximum 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 occasionally, so should not be
       very big.

       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.

   Validity of UTF-8 strings

       When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and
       subjects are (by default) checked for validity on entry to the relevant
       functions. From release 7.3 of PCRE, the check is according  the  rules
       of  RFC  3629, which are themselves derived from the Unicode specifica-
       tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which
       allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current
       check allows only values in the range U+0 to U+10FFFF, excluding U+D800
       to U+DFFF.

       The  excluded  code  points are the "Low Surrogate Area" of Unicode, of
       which the Unicode Standard says this: "The Low Surrogate Area does  not
       contain  any  character  assignments,  consequently  no  character code
       charts or namelists are provided for this area. Surrogates are reserved
       for  use  with  UTF-16 and then must be used in pairs." The code points
       that are encoded by UTF-16 pairs  are  available  as  independent  code
       points  in  the  UTF-8  encoding.  (In other words, the whole surrogate
       thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)

       If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return
       (PCRE_ERROR_BADUTF8) 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  when  PCRE_NO_UTF8_CHECK  is  set,
       what  happens  depends on why the string is invalid. If the string con-
       forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
       string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,
       apart from the initial validity test, PCRE (when in UTF-8 mode) handles
       strings  according  to  the more liberal rules of RFC 2279. However, if
       the string does not even conform to RFC 2279, the result is  undefined.
       Your program may crash.

       If  you  want  to  process  strings  of  values  in the full range 0 to
       0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
       set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
       this situation, you will have to apply your own validity check.

   General comments about UTF-8 mode

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

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

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

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

       5. 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().

       6. 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}.

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

       8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
       escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
       acters.

       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 3QH, 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  two  initials,
       followed by the two digits 10, at the domain cam.ac.uk.


REVISION

       Last updated: 12 April 2008
       Copyright (c) 1997-2008 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. It assumes use of the  configure
       script,  where the optional features are selected or deselected by pro-
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of  CMakeSetup  if  you  are  using
       CMake instead of configure to build PCRE.

       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  include  descriptions  of 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  complemen-
       tary  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 30K  of  tables  to  the
       PCRE  library.  Only  the general category 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. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which  causes  PCRE  to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

         --enable-newline-is-any

       causes PCRE to recognize any Unicode newline sequence.

       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.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line  ending  sequence.  If
       you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.


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.


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. By default these point to malloc() and free(), but  you
       can replace the pointers so that your own functions are used.

       Separate  functions  are  provided  rather  than  using pcre_malloc and
       pcre_free 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  malloc()  and free(). 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.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE  uses fixed tables for processing characters whose code values are
       less than 256. By default, PCRE is built with a set of tables that  are
       distributed  in  the  file pcre_chartables.c.dist. These tables are for
       ASCII codes only. If you add

         --enable-rebuild-chartables

       to the configure command, the distributed tables are  no  longer  used.
       Instead,  a  program  called dftables is compiled and run. This outputs
       the source for new set of tables, created in the default locale of your
       C runtime system. (This method of replacing the tables does not work if
       you are cross compiling, because dftables is run on the local host.  If
       you  need  to  create alternative tables when cross compiling, you will
       have to do so "by hand".)


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).
       This is the case for most computer operating systems.  PCRE  can,  how-
       ever, be compiled to run in an EBCDIC environment by adding

         --enable-ebcdic

       to the configure command. This setting implies --enable-rebuild-charta-
       bles. You should only use it if you know that  you  are  in  an  EBCDIC
       environment (for example, an IBM mainframe operating system).


PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of

         --enable-pcregrep-libz
         --enable-pcregrep-libbz2

       to the configure command. These options naturally require that the rel-
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.


PCRETEST OPTION FOR LIBREADLINE SUPPORT

       If you add

         --enable-pcretest-libreadline

       to  the  configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licenced, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.

       Setting  this  option  causes  the -lreadline option to be added to the
       pcretest build. In many operating environments with  a  sytem-installed
       libreadline this is sufficient. However, in some environments (e.g.  if
       an unmodified distribution version of readline is in use),  some  extra
       configuration  may  be necessary. The INSTALL file for libreadline says
       this:

         "Readline uses the termcap functions, but does not link with the
         termcap or curses library itself, allowing applications which link
         with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


SEE ALSO

       pcreapi(3), pcre_config(3).


AUTHOR

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


REVISION

       Last updated: 13 April 2008
       Copyright (c) 1997-2008 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 alternative 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 ALTERNATIVE MATCHING ALGORITHM

       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, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       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 alternative 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. However, pos-
       sessive quantifiers can make a difference when what follows could  also
       match what is quantified, for example in a pattern like this:

         ^a++\w!

       This  pattern matches "aaab!" but not "aaa!", which would be matched by
       a non-possessive quantifier. Similarly, if an atomic group is  present,
       it  is matched as if it were a standalone pattern at the current point,
       and the longest match is then "locked in" for the rest of  the  overall
       pattern.

       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 or test for a specific group recursion are not
       supported.

       5. Because many paths through the tree may be  active,  the  \K  escape
       sequence, which resets the start of the match when encountered (but may
       be on some paths and not on others), is not  supported.  It  causes  an
       error if encountered.

       6.  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.

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

       8.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
       are not supported. (*FAIL) is supported, and  behaves  like  a  failing
       negative assertion.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using  the alternative matching algorithm provides the following advan-
       tages:

       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 alternative algorithm.
       For non-anchored patterns, the starting position of a partial match  is
       available.

       3.  Because  the  alternative  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 partial matching each time.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative 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. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

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


REVISION

       Last updated: 19 April 2008
       Copyright (c) 1997-2008 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
       are also some 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
       compile and 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), and scans the subject just once. 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 documentation.

       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 five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each  of  the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE is built, a  default
       can  be  specified.  The default default is LF, which is the Unix stan-
       dard. When PCRE is run, the default can be overridden,  either  when  a
       pattern is compiled, or when it is matched.

       At compile time, the newline convention can be specified by the options
       argument of pcre_compile(), or it can be specified by special  text  at
       the start of the pattern itself; this overrides any other settings. See
       the pcrepattern page for details of the special character sequences.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter  or pair of characters that indicate a line break". The choice of
       newline convention affects the handling of  the  dot,  circumflex,  and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre_exec() options below.

       The choice of newline convention does not affect the interpretation  of
       the  \n  or  \r  escape  sequences, nor does it affect what \R matches,
       which is controlled in a similar way, but by separate options.


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.  However, compiling a regular expression
       with one version of PCRE for use with a different version is not  guar-
       anteed to work and may cause crashes.


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 four values  that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and -1 for ANY. The default should normally be  the  standard  sequence
       for your operating system.

         PCRE_CONFIG_BSR

       The output is an integer whose value indicates what character sequences
       the \R escape sequence matches by default. A value of 0 means  that  \R
       matches  any  Unicode  line ending sequence; a value of 1 means that \R
       matches only CR, LF, or CRLF. The default can be overridden when a pat-
       tern is compiled or matched.

         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 various bit settings that affect the com-
       pilation.  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_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. The default is specified when
       PCRE is built. It can be overridden from within the pattern, or by set-
       ting an option when a compiled pattern is matched.

         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
       newline characters, 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_JAVASCRIPT_COMPAT

       If this option is set, PCRE's behaviour is changed in some ways so that
       it is compatible with JavaScript rather than Perl. The changes  are  as
       follows:

       (1)  A  lone  closing square bracket in a pattern causes a compile-time
       error, because this is illegal in JavaScript (by default it is  treated
       as a data character). Thus, the pattern AB]CD becomes illegal when this
       option is set.

       (2) At run time, a back reference to an unset subpattern group  matches
       an  empty  string (by default this causes the current matching alterna-
       tive to fail). A pattern such as (\1)(a) succeeds when this  option  is
       set  (assuming  it can find an "a" in the subject), whereas it fails by
       default, for Perl compatibility.

         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
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       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 PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
       two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
       U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
       (paragraph  separator,  U+2029).  The  last  two are recognized only in
       UTF-8 mode.

       The newline setting in the  options  word  uses  three  bits  that  are
       treated as a number, giving eight possibilities. Currently only six are
       used (default plus the five values above). This means that if  you  set
       more  than one newline option, the combination may or may not be sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
       cause an error.

       The only time that a line break is specially recognized when  compiling
       a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
       character class is encountered. This indicates  a  comment  that  lasts
       until  after the next line break sequence. In other circumstances, line
       break  sequences  are  treated  as  literal  data,   except   that   in
       PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
       and are therefore ignored.

       The newline option that is 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.  There  is  a  discussion about the validity of
       UTF-8 strings in the main pcre page. 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 perfor-
       mance  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 checking 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. As PCRE has developed, some error codes have
       fallen out of use. To avoid confusion, they have not been re-used.

          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  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (? or (?-
         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  [this code is not in use]
         20  regular expression is 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  [this code is not in use]
         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 in subpattern name (missing terminator)
         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 10000)
         50  [this code is not in use]
         51  octal value is greater than \377 (not in UTF-8 mode)
         52  internal error: overran compiling workspace
         53  internal  error:  previously-checked  referenced  subpattern  not
       found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options
         57  \g is not followed by a braced, angle-bracketed, or quoted
               name/number or by a plain number
         58  a numbered reference must not be zero
         59  (*VERB) with an argument is not supported
         60  (*VERB) not recognized
         61  number is too big
         62  subpattern name expected
         63  digit expected after (?+
         64  ] is an invalid data character in JavaScript compatibility mode

       The  numbers  32  and 10000 in errors 48 and 49 are defaults; different
       values may be used if the limits were changed when PCRE was built.


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. If you are handling characters with codes  greater
       than  128, you should either use UTF-8 and Unicode, or use locales, but
       not try to mix the two.

       PCRE contains an internal set of tables that are used  when  the  final
       argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

       The  internal tables can always be overridden by tables supplied by the
       application that calls PCRE. These may be created in a different locale
       from  the  default.  As more and more applications change to using Uni-
       code, 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);

       The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale is "french".

       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), its value is returned. 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_HASCRORLF

       Return  1  if  the  pattern  contains any explicit matches for CR or LF
       characters, otherwise 0. The fourth argument should  point  to  an  int
       variable.  An explicit match is either a literal CR or LF character, or
       \r or \n.

         PCRE_INFO_JCHANGED

       Return 1 if the (?J) or (?-J) option setting is used  in  the  pattern,
       otherwise  0. The fourth argument should point to an int variable. (?J)
       and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

         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):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<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_OKPARTIAL

       Return 1 if the pattern can be used for partial matching, otherwise  0.
       The  fourth  argument  should point to an int variable. The pcrepartial
       documentation lists the restrictions that apply to patterns  when  par-
       tial matching is used.

         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 at the start of the pattern itself. In
       other  words,  they are the options that will be in force when matching
       starts. For example, if the pattern /(?im)abc(?-i)d/ is  compiled  with
       the  PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
       and PCRE_EXTENDED.

       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_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. These  options  override  the
       choice that was made or defaulted when the pattern was compiled.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These  options  override  the  newline  definition  that  was chosen or
       defaulted when the pattern was compiled. For details, see the  descrip-
       tion  of  pcre_compile()  above.  During  matching,  the newline choice
       affects the behaviour of the dot, circumflex,  and  dollar  metacharac-
       ters.  It may also alter the way the match position is advanced after a
       match failure for an unanchored pattern.

       When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY  is
       set,  and a match attempt for an unanchored pattern fails when the cur-
       rent position is at a  CRLF  sequence,  and  the  pattern  contains  no
       explicit  matches  for  CR  or  LF  characters,  the  match position is
       advanced by two characters instead of one, in other words, to after the
       CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected. For example, if the  pattern  is  .+A  (and  the  PCRE_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing at the start, it skips both the CR and the LF before  retrying.
       However,  the  pattern  [\r\n]A does match that string, because it con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those characters, or one of the \r or  \n  escape  sequences.  Implicit
       matches  such  as [^X] do not count, nor does \s (which includes CR and
       LF in the characters that it matches).

       Notwithstanding the above, anomalous effects may still occur when  CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.

         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. There is a discussion about
       the validity of UTF-8 strings in the section on UTF-8  support  in  the
       main  pcre  page.  If  an  invalid  UTF-8  sequence  of bytes is found,
       pcre_exec() returns the error PCRE_ERROR_BADUTF8. If  startoffset  con-
       tains 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_OPCODE (-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_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.

         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_BADNEWLINE     (-23)

       An invalid combination of PCRE_NEWLINE_xxx options was given.

       Error numbers -16 to -20 and -22 are not used by pcre_exec().


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 these error codes:

         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 the
       error code

         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(?<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. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
       behaviour may not be what you want (see the next section).


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, PCRE_ERROR_NOSUBSTRING
       (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
       function  returns one of the numbers 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  (-7)  if
       there  are none. The format of the table is described above in the sec-
       tion entitled Information about a  pattern.   Given  all  the  relevant
       entries  for the name, you can extract each of their numbers, and hence
       the captured 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 matching algorithm that  scans  the
       subject  string  just  once, and does not backtrack. This has different
       characteristics to the normal algorithm, and  is  not  compatible  with
       Perl.  Some  of the features of PCRE patterns are not supported. Never-
       theless, there are times when this kind of matching can be useful.  For
       a discussion of the two matching algorithms, see the pcrematching docu-
       mentation.

       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 alterna-
       tive  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.  In
       fact,  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
       that  uses  a back reference for the condition, or a test for recursion
       in a specific group. These are 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.


SEE ALSO

       pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
       tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).


AUTHOR

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


REVISION

       Last updated: 12 April 2008
       Copyright (c) 1997-2008 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)abc(?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 normally contains the offset within  the  subject
       at  which  the  current  match  attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect  the
       modified  starting  point.  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.


AUTHOR

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


REVISION

       Last updated: 29 May 2007
       Copyright (c) 1997-2007 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  mainly
       with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
       some features that are expected to be in the forthcoming Perl 5.10.

       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 (??{code})
       constructions. However, there is support for recursive  patterns.  This
       is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
       "callout" feature allows an external function to be called during  pat-
       tern matching. See the pcrecallout documentation for details.

       9.  Subpatterns  that  are  called  recursively or as "subroutines" are
       always treated as atomic groups in  PCRE.  This  is  like  Python,  but
       unlike Perl.

       10.  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".

       11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
       (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
       the forms without an  argument.  PCRE  does  not  support  (*MARK).  If
       (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-
       ture group; this is different to Perl.

       12. PCRE provides some extensions to the Perl regular expression facil-
       ities.   Perl  5.10  will  include new features that are not in earlier
       versions, some of which (such as named parentheses) have been  in  PCRE
       for some time. This list is with respect to Perl 5.10:

       (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 quietly
       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 escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

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

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

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

       (l)  PCRE  recognizes some special sequences such as (*CR) at the start
       of a pattern that set overall options that cannot be changed within the
       pattern.


AUTHOR

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


REVISION

       Last updated: 11 September 2007
       Copyright (c) 1997-2007 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 that are supported
       by PCRE are described in detail below. There is a quick-reference  syn-
       tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular  expression  syntax (which does not conflict with the Perl syn-
       tax) in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered 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  description  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. Some of the features discussed below are not available
       when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
       alternative function, and how it differs from the normal function,  are
       discussed in the pcrematching page.


NEWLINE CONVENTIONS

       PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The pcreapi page  has  further
       discussion  about newlines, and shows how to set the newline convention
       in the options arguments for the compiling and matching functions.

       It is also possible to specify a newline convention by starting a  pat-
       tern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to pcre_compile(). For
       example, on a Unix system where LF is the default newline sequence, the
       pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. Note that these special settings,  which  are  not
       Perl-compatible,  are  recognized  only at the very start of a pattern,
       and that they must be in upper case.  If  more  than  one  of  them  is
       present, the last one is used.

       The  newline  convention  does  not  affect what the \R escape sequence
       matches. By default, this is any Unicode  newline  sequence,  for  Perl
       compatibility.  However, this can be changed; see the description of \R
       in the section entitled "Newline sequences" below. A change of \R  set-
       ting can be combined with a change of newline convention.


CHARACTERS AND METACHARACTERS

       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  within 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        linefeed (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 value in hexadecimal is 7FFFFFFF. Note that this is bigger
       than the largest Unicode code point, which is 10FFFF.

       If characters other than hexadecimal digits appear between \x{  and  },
       or if there is no terminating }, 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, and 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 sequences \R and \X are interpreted as the characters  "R"
       and  "X", respectively. Outside a character class, these sequences have
       different meanings (see below).

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

   Generic character types

       Another 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
         \h     any horizontal whitespace character
         \H     any character that is not a horizontal whitespace character
         \s     any whitespace character
         \S     any character that is not a whitespace character
         \v     any vertical whitespace character
         \V     any character that is not a vertical 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.

       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.  These  sequences  retain
       their original meanings from before UTF-8 support was available, mainly
       for efficiency reasons.

       The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
       the  other  sequences, these do match certain high-valued codepoints in
       UTF-8 mode.  The horizontal space characters are:

         U+0009     Horizontal tab
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed
         U+000B     Vertical tab
         U+000C     Formfeed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

       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 a French locale such  as  "fr_FR"  in  Unix-like
       systems,  or "french" in Windows, some character codes greater than 128
       are used for accented letters, and these are matched by \w. The use  of
       locales with Unicode is discouraged.

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
       mode \R is equivalent to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This  is  an  example  of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

       In UTF-8 mode, two additional characters whose codepoints  are  greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode character property support is not  needed  for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when  PCRE  is  built;  if this is the case, the other behaviour can be
       requested via the PCRE_BSR_UNICODE option.   It  is  also  possible  to
       specify  these  settings  by  starting a pattern string with one of the
       following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to pcre_compile(), but
       they can be overridden by options given to pcre_exec(). Note that these
       special settings, which are not Perl-compatible, are recognized only at
       the  very  start  of a pattern, and that they must be in upper case. If
       more than one of them is present, the last one is  used.  They  can  be
       combined  with  a  change of newline convention, for example, a pattern
       can start with:

         (*ANY)(*BSR_ANYCRLF)

       Inside a character class, \R matches the letter "R".

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional  escape sequences that match characters with specific properties
       are available.  When not in UTF-8 mode, these sequences are  of  course
       limited  to  testing characters whose codepoints are less than 256, but
       they do work in this mode.  The extra escape sequences 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,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
       Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
       Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
       gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,
       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
       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