pcre(3)
PCRE(3) C LIBRARY FUNCTIONS PCRE(3)
NAME
PCRE - Perl-compatible regular expressions (original API)
PLEASE TAKE NOTE
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 pro-
jects are advised to use the new PCRE2 libraries.
INTRODUCTION
The PCRE library is a set of functions that implement regu-
lar 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 syn-
tax 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 sup-
ports 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
PCRE 8.37 Last change: 10 February 2015 1
PCRE(3) C LIBRARY FUNCTIONS PCRE(3)
correspond to Unicode release 6.3.0. In addition to the
Perl-compatible matching function, PCRE contains an alterna-
tive 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:
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 possi-
ble 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 sys-
tems 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 charac-
ters 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
PCRE 8.37 Last change: 10 February 2015 2
PCRE(3) C LIBRARY FUNCTIONS PCRE(3)
options for UTF. Alternatively, from release 8.33, you can
set the PCRE_NEVER_UTF option at compile time. This causes
an 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 subse-
quent 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 dif-
ferent 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 optimiza-
tion support
pcrelimits details of size and other limits
pcrematching discussion of the two matching algo-
rithms
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
PCRE 8.37 Last change: 10 February 2015 3
PCRE(3) C LIBRARY FUNCTIONS PCRE(3)
precompiled patterns
pcresample discussion of the pcredemo program
pcrestack discussion of stack usage
pcresyntax quick syntax reference
pcretest description of the pcretest testing com-
mand
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
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: 10 February 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE 8.37 Last change: 10 February 2015 4
Man(1) output converted with
man2html