PCRECOMPAT(3)						     Library Functions Manual						     PCRECOMPAT(3)

NAME

       PCRE - Perl-compatible regular expressions

DIFFERENCES BETWEEN PCRE AND PERL


       This  document  describes  the  differences  in	the ways that PCRE and Perl handle regular expressions. The differences described here are
       mainly with respect to Perl 5.8, though PCRE versions 7.0 and later contain some features that are expected to be in the  forthcoming  Perl
       5.10.

       1.  PCRE  has  only a subset of Perl's UTF-8 and Unicode 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  exam-
       ple, (?!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  suc-
       ceeding), 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  can be used in the pattern to represent a binary zero.

       5. The following Perl escape sequences are not supported: l, u, L, U, and N. 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.  The	Perl  escape  sequences p, P, and X are supported only if PCRE is built with Unicode character property support. The properties
       that can be tested with p and P are limited to the general category properties such as Lu and Nd, script names such as Greek or Han,  and
       the derived properties Any and L&.

       7.  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$xyzE	      abc$xyz		abc followed by the
						  contents of $xyz
	   Qabc$xyzE       abc$xyz		abc$xyz
	   QabcE$QxyzE   abc$xyz		abc$xyz

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

       8.  Fairly  obviously,  PCRE does not support the (?{code}) and (??{code}) constructions. However, there is support for recursive patterns.
       This is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout" feature allows an external function to be called dur-
       ing pattern matching. See the pcrecallout documentation for details.

       9. Subpatterns that are called recursively or as "subroutines" are always treated as atomic groups in PCRE. This is like Python, but unlike
       Perl.

       10. 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".

       11.  PCRE  does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the
       forms without an argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing parentheses, PCRE  does  not	set  that  capture
       group; this is different to Perl.

       12.  PCRE  provides some extensions to the Perl regular expression facilities.  Perl 5.10 will include new features that are not in earlier
       versions, some of which (such as named parentheses) have been in PCRE for some time. This list is with respect to Perl 5.10:

       (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. Otherwise, like Perl, the backslash is qui-
       etly ignored.  (Perl can be made to issue a warning.)

       (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers 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 at matching time 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 escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on different hosts that have the other endianness.

       (k) The alternative matching function (pcre_dfa_exec()) matches in a different way and is not Perl-compatible.

       (l)  PCRE  recognizes some special sequences such as (*CR) at the start of a pattern that set overall options that cannot be changed within
       the pattern.

AUTHOR


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION


       Last updated: 11 September 2007
       Copyright (c) 1997-2007 University of Cambridge.

																     PCRECOMPAT(3)