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+This page is part of the PCRE2 HTML documentation. It was generated
+automatically from the original man page. If there is any nonsense in it,
+please consult the man page, in case the conversion went wrong.
+
+
+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. +
++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 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
+
+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. +
+
+Last updated: 11 July 2018
+
+Copyright © 1997-2018 University of Cambridge.
+
+
+Return to the PCRE2 index page. +