Man Linux: Main Page and Category List


       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 5.10.

       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  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
       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
       succeeding), 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 \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-
       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
       somewhat 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$xyz\E        abc$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

       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  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).

       12.  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  pattern
       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.

       13. PCRE provides  some  extensions  to  the  Perl  regular  expression
       facilities.   Perl  5.10  includes new features that are not in earlier
       versions 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

       (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
       quietly 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 followed 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.

       and   PCRE_NO_AUTO_CAPTURE   options   for  pcre_exec()  have  no  Perl

       (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


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


       Last updated: 04 October 2009
       Copyright (c) 1997-2009 University of Cambridge.