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 5.8.

       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  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
       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 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, and \N. In fact these are implemented by Perl's general string-han-
       dling 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 prop-
       erties 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. 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
       ities:

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

       (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 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_CAP-
       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.

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

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

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

Last updated: 06 June 2006
Copyright (c) 1997-2006 University of Cambridge.



                                                                 PCRECOMPAT(3)