/usr/man2/cat.3/pcrecompat.3.Z

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 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
       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.

       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
       classes.

       8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
       constructions.  However,  there is 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 pat-
       tern matching. See the pcrecallout documentation for details.

       9. 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".

       10. PCRE provides some extensions to the Perl regular expression facil-
       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.

       (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: 28 February 2005
Copyright (c) 1997-2005 University of Cambridge.

                                                                 PCRECOMPAT(3)