Index for Section 3

Alphabetical listing for P

Bottom of page

PCRE(3)
NAME
  PCRE - Perl-compatible regular expressions

DESCRIPTION
  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.

USER DOCUMENTATION
  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.

LIMITATIONS
  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.

UTF-8 SUPPORT
  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.

AUTHOR
  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.

Index for Section 3

Alphabetical listing for P

Top of page