pcrecompat man page on Aros

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PCRE(3)								       PCRE(3)

       PCRE - Perl-compatible regular expressions


       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 asser‐
       tions 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 succeed‐
       ing),  but  only	 if the negative lookahead assertion contains just one

       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 nor‐
       mal 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. Charac‐
       ters  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

       7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
       constructions. However, there is some experimental support  for	recur‐
       sive  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 facili‐

       (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 spe‐
       cial meaning is faulted.

       (d)  If	PCRE_UNGREEDY is set, the greediness of the repetition quanti‐
       fiers is inverted, that is, by default they are not greedy, but if fol‐
       lowed 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.

       TURE 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.


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