PCRE - Perl-compatible regular expressions

The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl, with just a few differences. The current implementation of PCRE (release 6.x) corresponds approximately with Perl 5.8, including support for UTF-8 encoded strings and Unicode general category properties. However, this support has to be explicitly enabled; it is not the default.

In addition to the Perl-compatible matching function, PCRE also contains an alternative matching 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. 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.

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 themselves are described in the pcrebuild page. Documentation about building PCRE for various operating systems can be found in the README file in the source distribution.

The library 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_", 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.

The user documentation for PCRE 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, all the sections are concatenated, for ease of searching. The sections are as follows: pcre this document 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 pcregrep description of the pcregrep command 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 pcreprecompile details of saving and re-using precompiled patterns pcresample discussion of the sample program pcretest description of the pcretest testing command In addition, in the "man" and HTML formats, there is a short page for each C library function, listing its arguments and results.

There are some size limitations in PCRE but it is hoped that they will never in practice be relevant.

The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is compiled with the default internal linkage size of 2. If you want to process regular expressions that are truly enormous, you can compile PCRE with an internal linkage size of 3 or 4 (see the README file in the source distribution and the pcrebuild documentation for details). In these cases the limit is substantially larger. However, the speed of execution will be slower.

All values in repeating quantifiers must be less than 65536. The maximum number of capturing subpatterns is 65535.

There is no limit to the number of non-capturing subpatterns, but the maximum depth of nesting of all kinds of parenthesized subpattern, including capturing subpatterns, assertions, and other types of subpattern, is 200.

The maximum length of a subject string is the largest positive number that an integer variable can hold. However, when using the traditional matching function, PCRE uses recursion to handle subpatterns and indefinite repetition. This means that the available stack space may limit the size of a subject string that can be processed by certain patterns.

From release 3.3, PCRE has had some support for character strings encoded in the UTF-8 format. For release 4.0 this was greatly extended to cover most common requirements, and in release 5.0 additional support for Unicode general category properties was added.

In order process UTF-8 strings, you must build PCRE to include UTF-8 support in the code, and, in addition, you must call pcre_compile() with the PCRE_UTF8 option flag. When you do this, both the pattern and any subject strings that are matched against it are treated as UTF-8 strings instead of just strings of bytes.

If you compile PCRE with UTF-8 support, but do not use it at run time, the library will be a bit bigger, but the additional run time overhead is limited to testing the PCRE_UTF8 flag in several places, so should not be very large.

If PCRE is built with Unicode character property support (which implies UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are supported. The available properties that can be tested are limited to the general category properties such as Lu for an upper case letter or Nd for a decimal number. A full list is given in the pcrepattern documentation. The PCRE library is increased in size by about 90K when Unicode property support is included.

The following comments apply when PCRE is running in UTF-8 mode:

1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects are checked for validity on entry to the relevant functions. If an invalid UTF-8 string is passed, an error return is given. In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respectively) contains only valid UTF-8 codes. In this case, it does not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program may crash.

2. In a pattern, the escape sequence \x{...}, where the contents of the braces is a string of hexadecimal digits, is interpreted as a UTF-8 character whose code number is the given hexadecimal number, for example: \x{1234}. If a non-hexadecimal digit appears between the braces, the item is not recognized. This escape sequence can be used either as a literal, or within a character class.

3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8 character if the value is greater than 127.

4. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes, for example: \x{100}{3}.

5. The dot metacharacter matches one UTF-8 character instead of a single byte.

6. The escape sequence \C can be used to match a single byte in UTF-8 mode, but its use can lead to some strange effects. This facility is not available in the alternative matching function, pcre_dfa_exec().

7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but the characters that PCRE recognizes as digits, spaces, or word characters remain the same set as before, all with values less than 256. This remains true even when PCRE includes Unicode property support, because to do otherwise would slow down PCRE in many common cases. If you really want to test for a wider sense of, say, "digit", you must use Unicode property tests such as \p{Nd}.

8. Similarly, characters that match the POSIX named character classes are all low-valued characters.

9. Case-insensitive matching applies only to characters whose values are less than 128, unless PCRE is built with Unicode property support. Even when Unicode property support is available, PCRE still uses its own character tables when checking the case of low-valued characters, so as not to degrade performance. The Unicode property information is used only for characters with higher values.

Philip Hazel

University Computing Service,

Cambridge CB2 3QG, 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 initial and surname, separated by a dot, at the domain ucs.cam.ac.uk. Last updated: 07 March 2005

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