PCRE(3) - phpMan

PCRE(3) Library Functions Manual 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 compati-
bility.

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 pos-
sible was done by Zoltan Herczeg.

Starting with release 8.32 it is possible to compile a third separate PCRE library, which
supports 32-bit character strings (including UTF-32 strings). The build process allows any
set of the 8-, 16- and 32-bit libraries. The work to make this possible was done by Chris-
tian 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 documenta-
tion 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 6.2.0.

In addition to the Perl-compatible matching function, PCRE contains an alternative func-
tion 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 algo-
rithms, 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:

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-AUTO-
TOOLS_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.

SECURITY CONSIDERATIONS

If you are using PCRE in a non-UTF application that permits users to supply arbitrary pat-
terns 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 inter-
prets 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 suffi-
ciently many resources as to cause your application to lose performance.

The best way of guarding against this possibility is to use the pcre_fullinfo() function
to check the compiled pattern's options for UTF.

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.

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
pcre16 details of the 16-bit library
pcre32 details of the 32-bit library
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 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 support
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 patterns
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 addition, in the "man" and HTML formats, there is a short page for each C library func-
tion, listing its arguments and results.

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

PCRE 8.32 11 November 2012 PCRE(3)

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