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. Some features that appeared in Python and PCRE before they appeared in Perl are also available using the Python syntax, there is some support for one or two .NET and Oniguruma syntax items, and there is an option for requesting some minor changes that give better JavaScript compatibility.
The current implementation of PCRE corresponds approximately with Perl 5.10, including support for UTF-8 encoded strings and Unicode general category properties. However, UTF-8 and Unicode support has to be explicitly enabled; it is not the default. The Unicode tables correspond to Unicode release 5.2.0.
In addition to the Perl-compatible matching function, PCRE 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 pcrematching(3) 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(3) 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. There is
a syntax summary in the pcresyntax page.
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(3) page.
Documentation about building PCRE for various operating
systems can be found in the README and NON-UNIX-USE files 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, except the pcredemo section, are concatenated, for
ease of searching. The sections are as follows:
this document
show PCRE installation configuration information
details of PCRE's native C API
options for building PCRE
details of the callout feature
discussion of Perl compatibility
details of the C++ wrapper
a demonstration C program that uses PCRE
description of the pcregrep command
discussion of the two matching algorithms
details of the partial matching facility
syntax and semantics of supported regular expressions
discussion of performance issues
the POSIX-compatible C API
details of saving and re-using precompiled patterns
discussion of the pcredemo program
discussion of stack usage
quick syntax reference
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(3) documentation
for details). In these cases the limit is substantially
larger. However, the speed of execution is slower.
All values in repeating quantifiers must be less than 65536.
There is no limit to the number of parenthesized subpatterns, but there can be no more than 65535 capturing subpatterns.
The maximum length of name for a named subpattern is 32 characters, and the maximum number of named subpatterns is 10000.
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. For a discussion of stack issues, see the pcrestack(3) documentation.
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()(3) with the PCRE_UTF8 option flag, or the pattern must start with the sequence (*UTF8). When either of these is the case, both the pattern and any subject strings that are matched against it are treated as UTF-8 strings instead of strings of 1-byte characters.
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 occasionally, so should not be very big.
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, the Unicode script names such as Arabic or Han, and the derived properties Any and L&. A full list is given in the pcrepattern(3) documentation. Only the short names for properties are supported. For example, \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE does not support this.
When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects are (by default) checked for validity on entry to the relevant functions. From release 7.3 of PCRE, the check is according the rules of RFC 3629, which are themselves derived from the Unicode specification. Earlier releases of PCRE followed the rules of RFC 2279, which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.
The excluded code points are the "Low Surrogate Area" of Unicode, of which the Unicode Standard says this: "The Low Surrogate Area does not contain any character assignments, consequently no character code charts or namelists are provided for this area. Surrogates are reserved for use with UTF-16 and then must be used in pairs." The code points that are encoded by UTF-16 pairs are available as independent code points in the UTF-8 encoding. (In other words, the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
If an invalid UTF-8 string is passed to PCRE, an error return (PCRE_ERROR_BADUTF8) 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 when PCRE_NO_UTF8_CHECK is set, what happens depends on why the string is invalid. If the string conforms to the "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity test, PCRE (when in UTF-8 mode) handles strings according to the more liberal rules of RFC 2279. However, if the string does not even conform to RFC 2279, the result is undefined. Your program may crash.
If you want to process strings of values in the full range 0 to 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this situation, you will have to apply your own validity check.
1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte UTF-8 character if the value is greater than 127.
2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 characters for values greater than \177.
3. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes, for example: \x{100}{3}.
4. The dot metacharacter matches one UTF-8 character instead of a single byte.
5. 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().
6. 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}. Note that this also applies to \b, because it is defined in terms of \w and \W.
7. Similarly, characters that match the POSIX named character classes are all low-valued characters.
8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode 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. Even when Unicode property support is available, PCRE supports case-insensitive matching only when there is a one-to-one mapping between a letter's cases. There are a small number of many-to-one mappings in Unicode; these are not supported by PCRE.
Philip Hazel University Computing Service Cambridge CB2 3QH, 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 two initials, followed by the two digits 10, at the domain cam.ac.uk.
Last updated: 01 March 2010 Copyright (c) 1997-2010 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |