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 with
       respect to Perl versions 5.10 and above.

       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&. PCRE does support the Cs (surrogate) property, which Perl does not; the Perl documentation says "Because
       Perl  hides  the need for the user to understand the internal representation of Unicode characters, there is no need to implement the some-
       what messy concept of surrogates."

       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 it is in Perl 5.10. Also, the PCRE "callout" feature allows an external function to be called during
       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.  There  is  a  discussion	of  an	example that explains this in more detail in the section on recursion differences from Perl in the
       pcrepattern page.

       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's  handling  of duplicate subpattern numbers and duplicate subpattern names is not as general as Perl's. This is a consequence of
       the fact the PCRE works internally just with numbers, using an external table to translate between numbers and names. In particular, a pat-
       tern such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have the same number but different names, is not supported, and causes
       an error at compile time. If it were allowed, it would not be possible to distinguish which parentheses matched, because both names map	to
       capturing subpattern number 1. To avoid this confusing situation, an error is given at compile time.

       12. Perl recognizes comments in some places that PCRE doesn't, for example, between the ( and ? at the start of a subpattern.

       13.  PCRE provides some extensions to the Perl regular expression facilities.  Perl 5.10 includes new features that are not in earlier ver-
       sions of Perl, 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 in PCRE 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,  PCRE_NOTEMPTY_ATSTART,  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: 31 October 2010
       Copyright (c) 1997-2010 University of Cambridge.

																     PCRECOMPAT(3)