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. Some features that appeared in Python and PCRE
before they appeared in Perl are also available using the Python syn‐
tax, 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, 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 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 mainte‐
nance load, most of the documentation describes the 8-bit library, with
the differences for the 16-bit and 32-bit libraries described sepa‐
rately 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 function that matches the same compiled patterns in a dif‐
ferent 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 contribu‐
tions 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 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.
SECURITY CONSIDERATIONS
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 valid‐
ity. If the data string is very long, such a check might use suffi‐
ciently many resources as to cause your application to lose perfor‐
mance.
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 pro‐
vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea‐
ture in the pcreapi page.
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, except the pcredemo sec‐
tion, are concatenated, for ease of searching. The sections are as fol‐
lows:
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 function, 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
Last updated: 11 November 2012
Copyright (c) 1997-2012 University of Cambridge.
PCRE 8.32 11 November 2012 PCRE(3)
