focal (3) pcre2.3.gz
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
INTRODUCTION
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.
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 8-bit, 16-bit, or 32-bit code units, which means
that up to three separate libraries may be installed. 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.
SECURITY CONSIDERATIONS
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.
USER DOCUMENTATION
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.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
Putting an actual email address here is a spam magnet. 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: 17 September 2018
Copyright (c) 1997-2018 University of Cambridge.