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NAME

       PCRE - Perl-compatible regular expressions

INTRODUCTION


       The  PCRE  library  is  a  set  of  functions  that  implement  regular
       expression pattern matching using the  same  syntax  and  semantics  as
       Perl,  with  just  a  few  differences.  Some features that appeared in
       Python and PCRE before they appeared in Perl are also  available  using
       the  Python  syntax,  there  is  some  support  for one or two .NET and
       Oniguruma syntax items, and there is  an  option  for  requesting  some
       minor changes that give better JavaScript compatibility.

       The  current implementation of PCRE corresponds approximately with Perl
       5.12, 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 5.2.0.

       In  addition to the Perl-compatible matching function, PCRE contains an
       alternative 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
       pcrepattern 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
       themselves are described in the  pcrebuild  page.  Documentation  about
       building  PCRE for various operating systems can be found in the README
       and NON-UNIX-USE files 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
       sections.  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, except the pcredemo
       section, 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
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the pcredemo program
         pcrestack         discussion of stack usage
         pcresyntax        quick syntax reference
         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
       indefinite 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
       support 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, or the  pattern  must  start  with  the
       sequence  (*UTF8).  When  either of these is the case, both the pattern
       and any subject strings that are matched  against  it  are  treated  as
       UTF-8 strings instead of strings of 1-byte characters.

       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
       supported.  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
       supported.  For  example,  \p{L}  matches  a  letter. Its Perl synonym,
       \p{Letter}, 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
       specification. 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
       conforms 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
       individual bytes, for example: \x{100}{3}.

       4.  The  dot  metacharacter  matches  one  UTF-8 character instead of a
       single 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, by default, the characters that
       PCRE recognizes as digits, spaces, or word characters remain  the  same
       set  as  before,  all with values less than 256. This remains true even
       when PCRE is built to include Unicode property support, because  to  do
       otherwise would slow down PCRE in many common cases. Note in particular
       that this applies to \b and \B, because they are defined in terms of \w
       and  \W. If you really want to test for a wider sense of, say, "digit",
       you  can  use  explicit  Unicode  property  tests   such   as   \p{Nd}.
       Alternatively,  if  you  set  the  PCRE_UCP  option,  the  way that the
       character escapes work is changed so that Unicode properties  are  used
       to  determine  which  characters  match.  There are more details in the
       section on generic character types in the pcrepattern documentation.

       7. Similarly, characters that match the POSIX named  character  classes
       are all low-valued characters, unless the PCRE_UCP option is set.

       8.  However,  the  horizontal  and vertical whitespace matching escapes
       (\h, \H, \v, and \V) do match all the appropriate  Unicode  characters,
       whether or not PCRE_UCP is set.

       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. Furthermore, 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 supported 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: 13 November 2010
       Copyright (c) 1997-2010 University of Cambridge.

                                                                       PCRE(3)