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+
+This page is part of the PCRE 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.
+
+
+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. Certain features that appeared in Python and PCRE before they +appeared in Perl are also available using the Python syntax. There is also some +support for certain .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 (release 7.x) 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 5.0.0. +
++In addition to the Perl-compatible matching function, PCRE 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. 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 +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 + pcre-config show PCRE installation configuration information + 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 + pcresyntax quick syntax reference + 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 + pcrestack discussion of stack usage + 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 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 +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() +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 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 +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}. +
++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: 12 April 2008
+
+Copyright © 1997-2008 University of Cambridge.
+
+
+Return to the PCRE index page. +