pcre(3) — Linux manual page


PCRE(3)                   Library Functions Manual                   PCRE(3)

NAME         top

       PCRE - Perl-compatible regular expressions (original API)

PLEASE TAKE NOTE         top

       This document relates to PCRE releases that use the original API,
       with library names libpcre, libpcre16, and libpcre32. January 2015
       saw the first release of a new API, known as PCRE2, with release
       numbers starting at 10.00 and library names libpcre2-8, libpcre2-16,
       and libpcre2-32. The old libraries (now called PCRE1) are still being
       maintained for bug fixes, but there will be no new development. New
       projects are advised to use the new PCRE2 libraries.

INTRODUCTION         top

       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.

       Starting with release 8.30, it is possible to compile two separate
       PCRE libraries: the original, which supports 8-bit character strings
       (including UTF-8 strings), and a second library that supports 16-bit
       character strings (including UTF-16 strings). The build process
       allows either one or both to be built. The majority of the work to
       make this possible was done by Zoltan Herczeg.

       Starting with release 8.32 it is possible to compile a third separate
       PCRE library that supports 32-bit character strings (including UTF-32
       strings). The build process allows any combination of the 8-, 16- and
       32-bit libraries. The work to make this possible was done by
       Christian Persch.

       The three libraries contain identical sets of functions, except that
       the names in the 16-bit library start with pcre16_ instead of pcre_,
       and the names in the 32-bit library start with pcre32_ instead of
       pcre_. To avoid over-complication and reduce the documentation
       maintenance load, most of the documentation describes the 8-bit
       library, with the differences for the 16-bit and 32-bit libraries
       described separately in the pcre16 and pcre32 pages. References to
       functions or structures of the form pcre[16|32]_xxx should be read as
       meaning "pcre_xxx when using the 8-bit library, pcre16_xxx when using
       the 16-bit library, or pcre32_xxx when using the 32-bit library".

       The current implementation of PCRE corresponds approximately with
       Perl 5.12, including support for UTF-8/16/32 encoded strings and
       Unicode general category properties. However, UTF-8/16/32 and Unicode
       support has to be explicitly enabled; it is not the default. The
       Unicode tables correspond to Unicode release 6.3.0.

       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 for the 8-bit
       library. 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:


       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-AUTOTOOLS_BUILD files in the source distribution.

       The libraries 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_" or "_pcre16_" or
       "_pcre32_", 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.


       If you are using PCRE in a non-UTF application that permits users to
       supply arbitrary patterns for compilation, you should be aware of a
       feature that allows users to turn on UTF support from within a
       pattern, provided that PCRE was built with UTF support. For example,
       an 8-bit pattern that begins with "(*UTF8)" or "(*UTF)" turns on
       UTF-8 mode, which interprets patterns and subjects as strings of
       UTF-8 characters instead of individual 8-bit characters.  This causes
       both the pattern and any data against which it is matched to be
       checked for UTF-8 validity. If the data string is very long, such a
       check might use sufficiently many resources as to cause your
       application to lose performance.

       One way of guarding against this possibility is to use the
       pcre_fullinfo() function to check the compiled pattern's options for
       UTF.  Alternatively, from release 8.33, you can set the
       PCRE_NEVER_UTF option at compile time. This causes a compile time
       error if a pattern contains a UTF-setting sequence.

       If your application is one that supports UTF, be aware that validity
       checking can take time. If the same data string is to be matched many
       times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the
       second and subsequent matches to save redundant checks.

       Another way that performance can be hit is by running a pattern that
       has a very large search tree against a string that will never match.
       Nested unlimited repeats in a pattern are a common example. PCRE
       provides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT
       feature in the pcreapi page.


       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, the descriptions of the
       pcregrep and pcretest programs are in files called pcregrep.txt and
       pcretest.txt, respectively. The remaining sections, except for the
       pcredemo section (which is a program listing), are concatenated in
       pcre.txt, for ease of searching. The sections are as follows:

         pcre              this document
         pcre-config       show PCRE installation configuration information
         pcre16            details of the 16-bit library
         pcre32            details of the 32-bit library
         pcreapi           details of PCRE's native C API
         pcrebuild         building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper for the 8-bit library
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command (8-bit only)
         pcrejit           discussion of the just-in-time optimization
         pcrelimits        details of size and other limits
         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 for the 8-bit library
         pcreprecompile    details of saving and re-using precompiled
         pcresample        discussion of the pcredemo program
         pcrestack         discussion of stack usage
         pcresyntax        quick syntax reference
         pcretest          description of the pcretest testing command
         pcreunicode       discussion of Unicode and UTF-8/16/32 support

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

AUTHOR         top

       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         top

       Last updated: 10 February 2015
       Copyright (c) 1997-2015 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular Expressions)
       project.  Information about the project can be found at 
       ⟨http://www.pcre.org/⟩.  If you have a bug report for this manual
       page, see ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This
       page was obtained from the tarball pcre-8.44.tar.gz fetched from
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
       2020-09-18.  If you discover any rendering problems in this HTML ver‐
       sion of the page, or you believe there is a better or more up-to-date
       source for the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original man‐
       ual page), send a mail to man-pages@man7.org

PCRE 8.37                     10 February 2015                       PCRE(3)

Pages that refer to this page: egrep(1)fgrep(1)grep(1)pcre-config(1)pcretest(1)pcrepattern(3)pcresyntax(3)sefcontext_compile(8)