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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 4.x) corresponds approximately with Perl 5.8, including support for UTF-8 encoded strings. However, this support has to be explicitly enabled; it is not the default.
PCRE is written in C and released as a C library. However, a number of people have written wrappers and interfaces of various kinds. A C++ class is included in these contributions, which 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. Documentation about building PCRE for various operating systems can be found in the README file in the source distribution.
The user documentation for PCRE has been split up into 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 API
pcrebuild options for building PCRE
pcrecallout details of the callout feature
pcrecompat discussion of Perl compatibility
pcregrep description of the pcregrep command
pcrepattern syntax and semantics of supported
regular expressions
pcreperform discussion of performance issues
pcreposix the POSIX-compatible API
pcresample discussion of the sample program
pcretest the pcretest testing command
In addition, in the "man" and HTML formats, there is a short page for each 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). If 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, 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.
Starting at release 3.3, PCRE has had some support for character strings encoded in the UTF-8 format. For release 4.0 this has been greatly extended to cover most common requirements.
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.
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.
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.
8. Case-insensitive matching applies only to characters whose values are less than 256. PCRE does not support the notion of "case" for higher-valued characters.
9. PCRE does not support the use of Unicode tables and properties or the Perl escapes \p, \P, and \X.
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Last updated: 20 August 2003
Copyright (c) 1997-2003 University of Cambridge.
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