PCRE(3)                                                                PCRE(3)



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)
