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

DIFFERENCES FROM PERL
  This document describes the differences in the ways that PCRE and Perl
  handle regular expressions. The differences described here are with respect
  to Perl 5.8.

  1. PCRE does not have full UTF-8 support. Details of what it does have are
  given in the section on UTF-8 support in the main pcre page.

  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
  permits them, but they do not mean what you might think. For example,
  (?!a){3} does not assert that the next three characters are not "a". It
  just asserts that the next character is not "a" three times.

  3. Capturing subpatterns that occur inside negative lookahead assertions
  are counted, but their entries in the offsets vector are never set. Perl
  sets its numerical variables from any such patterns that are matched before
  the assertion fails to match something (thereby succeeding), but only if
  the negative lookahead assertion contains just one branch.

  4. Though binary zero characters are supported in the subject string, they
  are not allowed in a pattern string because it is passed as a normal C
  string, terminated by zero. The escape sequence "\0" can be used in the
  pattern to represent a binary zero.

  5. The following Perl escape sequences are not supported: \l, \u, \L, \U,
  \P, \p, \N, and \X. In fact these are implemented by Perl's general
  string-handling and are not part of its pattern matching engine. If any of
  these are encountered by PCRE, an error is generated.

  6. PCRE does support the \Q...\E escape for quoting substrings. Characters
  in between are treated as literals. This is slightly different from Perl in
  that $ and @ are also handled as literals inside the quotes. In Perl, they
  cause variable interpolation (but of course PCRE does not have variables).
  Note the following examples:

      Pattern		 PCRE matches	   Perl matches

      \Qabc$xyz\E	 abc$xyz	   abc followed by the
					     contents of $xyz
      \Qabc\$xyz\E	 abc\$xyz	   abc\$xyz
      \Qabc\E\$\Qxyz\E	 abc$xyz	   abc$xyz

  The \Q...\E sequence is recognized both inside and outside character
  classes.

  7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
  constructions. However, there is some experimental support for recursive
  patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the
  PCRE "callout" feature allows an external function to be called during
  pattern matching.

  8. There are some differences that are concerned with the settings of
  captured strings when part of a pattern is repeated. For example, matching
  "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE
  it is set to "b".

  9. PCRE provides some extensions to the Perl regular expression facilities:

  (a) Although lookbehind assertions must match fixed length strings, each
  alternative branch of a lookbehind assertion can match a different length
  of string. Perl requires them all to have the same length.

  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
  meta-character matches only at the very end of the string.

  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
  meaning is faulted.

  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers
  is inverted, that is, by default they are not greedy, but if followed by a
  question mark they are.

  (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the
  first matching position in the subject string.

  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
  options for pcre_exec() have no Perl equivalents.

  (g) The (?R), (?number), and (?P>name) constructs allows for recursive
  pattern matching (Perl can do this using the (?p{code}) construct, which
  PCRE cannot support.)

  (h) PCRE supports named capturing substrings, using the Python syntax.

  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's
  Java package.

  (j) The (R) condition, for testing recursion, is a PCRE extension.

  (k) The callout facility is PCRE-specific.

Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.

Index for Section 3

Alphabetical listing for P

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