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NAME | INTRODUCTION | SECURITY CONSIDERATIONS | USER DOCUMENTATION | AUTHOR | REVISION | COLOPHON |
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PCRE2(3) Library Functions Manual PCRE2(3)
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 is the name used for a revised API for the PCRE library,
which is a set of functions, written in C, that implement regular
expression pattern matching using the same syntax and semantics
as Perl, with just a few differences. After nearly two decades,
the limitations of the original API were making development
increasingly difficult. The new API is more extensible, and it
was simplified by abolishing the separate "study" optimizing
function; in PCRE2, patterns are automatically optimized where
possible. Since forking from PCRE1, the code has been extensively
refactored and new features introduced. The old library is now
obsolete and is no longer maintained.
As well as Perl-style regular expression patterns, some features
that appeared in Python and the original PCRE before they
appeared in Perl are available using the Python syntax. There is
also some support for one or two .NET and Oniguruma syntax items,
and there are options for requesting some minor changes that give
better ECMAScript (aka JavaScript) compatibility.
The source code for PCRE2 can be compiled to support strings of
8-bit, 16-bit, or 32-bit code units, which means that up to three
separate libraries may be installed, one for each code unit size.
The size of code unit is not related to the bit size of the
underlying hardware. In a 64-bit environment that also supports
32-bit applications, versions of PCRE2 that are compiled in both
64-bit and 32-bit modes may be needed.
The original work to extend PCRE to 16-bit and 32-bit code units
was done by Zoltan Herczeg and Christian Persch, respectively. In
all three cases, strings can be interpreted either as one
character per code unit, or as UTF-encoded Unicode, with support
for Unicode general category properties. Unicode support is
optional at build time (but is the default). However, processing
strings as UTF code units must be enabled explicitly at run time.
The version of Unicode in use can be discovered by running
pcre2test -C
The three libraries contain identical sets of functions, with
names ending in _8, _16, or _32, respectively (for example,
pcre2_compile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to
be 8, 16, or 32, a program that uses just one code unit width can
be written using generic names such as pcre2_compile(), and the
documentation is written assuming that this is the case.
In addition to the Perl-compatible matching function, PCRE2
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 pcre2matching page.
Details of exactly which Perl regular expression features are and
are not supported by PCRE2 are given in separate documents. See
the pcre2pattern and pcre2compat pages. There is a syntax summary
in the pcre2syntax page.
Some features of PCRE2 can be included, excluded, or changed when
the library is built. The pcre2_config() function makes it
possible for a client to discover which features are available.
The features themselves are described in the pcre2build page.
Documentation about building PCRE2 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 "_pcre2", 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 PCRE2 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. For example, an 8-bit pattern that begins with
"(*UTF)" turns on UTF-8 mode, which interprets patterns and
subjects as strings of UTF-8 code units 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
pcre2_pattern_info() function to check the compiled pattern's
options for PCRE2_UTF. Alternatively, you can set the
PCRE2_NEVER_UTF option when calling pcre2_compile(). This causes
a compile time error if the pattern contains a UTF-setting
sequence.
The use of Unicode properties for character types such as \d can
also be enabled from within the pattern, by specifying "(*UCP)".
This feature can be disallowed by setting the PCRE2_NEVER_UCP
option.
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 PCRE2_NO_UTF_CHECK option for
the second and subsequent matches to avoid running redundant
checks.
The use of the \C escape sequence in a UTF-8 or UTF-16 pattern
can lead to problems, because it may leave the current matching
point in the middle of a multi-code-unit character. The
PCRE2_NEVER_BACKSLASH_C option can be used by an application to
lock out the use of \C, causing a compile-time error if it is
encountered. It is also possible to build PCRE2 with the use of
\C permanently disabled.
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. PCRE2 provides some protection against this: see the
pcre2_set_match_limit() function in the pcre2api page. There is a
similar function called pcre2_set_depth_limit() that can be used
to restrict the amount of memory that is used.
The user documentation for PCRE2 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
pcre2grep and pcre2test programs are in files called
pcre2grep.txt and pcre2test.txt, respectively. The remaining
sections, except for the pcre2demo section (which is a program
listing), and the short pages for individual functions, are
concatenated in pcre2.txt, for ease of searching. The sections
are as follows:
pcre2 this document
pcre2-config show PCRE2 installation configuration
information
pcre2api details of PCRE2's native C API
pcre2build building PCRE2
pcre2callout details of the pattern callout feature
pcre2compat discussion of Perl compatibility
pcre2convert details of pattern conversion functions
pcre2demo a demonstration C program that uses PCRE2
pcre2grep description of the pcre2grep command (8-bit
only)
pcre2jit discussion of just-in-time optimization
support
pcre2limits details of size and other limits
pcre2matching discussion of the two matching algorithms
pcre2partial details of the partial matching facility
pcre2pattern syntax and semantics of supported regular
expression patterns
pcre2perform discussion of performance issues
pcre2posix the POSIX-compatible C API for the 8-bit
library
pcre2sample discussion of the pcre2demo program
pcre2serialize details of pattern serialization
pcre2syntax quick syntax reference
pcre2test description of the pcre2test command
pcre2unicode discussion of Unicode and UTF support
In the "man" and HTML formats, there is also a short page for
each C library function, listing its arguments and results.
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Putting an actual email address here is a spam magnet. If you
want to email me, use my two names separated by a dot at
gmail.com.
Last updated: 27 August 2021
Copyright (c) 1997-2021 University of Cambridge.
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 fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2024-06-14. If you
discover any rendering problems in this HTML version 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
manual page), send a mail to [email protected]
PCRE2 10.38 27 August 2021 PCRE2(3)
Pages that refer to this page: grep(1), pcre2-config(1), pcre2test(1), pcre2pattern(3), pcre2syntax(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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