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       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  versions  5.10  and

       1.  PCRE  has  only  a  subset of Perl's Unicode support. Details of what it does have are
       given in the pcreunicode page.

       2. PCRE allows repeat quantifiers only on parenthesized assertions, 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 (in principle:
       PCRE  optimizes  this  to  run the assertion just once). Perl allows repeat quantifiers on
       other assertions such as \b, but these do not seem to have any use.

       3. Capturing subpatterns that occur inside negative lookahead assertions are counted,  but
       their  entries  in  the offsets vector are never set. Perl sometimes (but not always) sets
       its numerical variables from inside negative assertions.

       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  when
       followed  by  a  character  name  or Unicode value. (\N on its own, matching a non-newline
       character, is supported.) 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 by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, \U
       and \u are interpreted as JavaScript interprets them.

       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 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  as subroutines (whether or not recursively) are always
       treated as atomic groups in PCRE. This is like Python, but unlike Perl.   Captured  values
       that  are  set  outside a subroutine call can be reference from inside in PCRE, but not in
       Perl. There is a discussion that explains these differences in more detail in the  section
       on recursion differences from Perl in the pcrepattern page.

       10.  If any of the backtracking control verbs are used in a subpattern that is called as a
       subroutine (whether or not recursively), their effect is confined to that  subpattern;  it
       does  not  extend  to  the  surrounding  pattern.  This is not always the case in Perl. In
       particular, if (*THEN) is present in a group that is called as a subroutine, its action is
       limited to that group, even if the group does not contain any | characters. Note that such
       subpatterns are processed as anchored at the point where they are tested.

       11. If a pattern contains more than one backtracking control verb, the first one  that  is
       backtracked  onto  acts.  For  example, in the pattern A(*COMMIT)B(*PRUNE)C a failure in B
       triggers (*COMMIT), but a failure  in  C  triggers  (*PRUNE).  Perl's  behaviour  is  more
       complex; in many cases it is the same as PCRE, but there are examples where it differs.

       12. Most backtracking verbs in assertions have their normal actions. They are not confined
       to the assertion.

       13. 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".

       14. 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.

       15. Perl recognizes comments in some places that PCRE does not, for example, between the (
       and ? at the start of a subpattern. If the /x modifier is set,  Perl  allows  white  space
       between  (  and ? (though current Perls warn that this is deprecated) but PCRE never does,
       even if the PCRE_EXTENDED option is set.

       16. Perl, when in warning mode, gives warnings for character classes  such  as  [A-\d]  or
       [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no warning features, so it
       gives an error in these cases because they are almost certainly user mistakes.

       17. In PCRE, the upper/lower case character properties Lu and Ll  are  not  affected  when
       case-independent  matching  is specified. For example, \p{Lu} always matches an upper case
       letter. I think Perl has changed in this respect; in the release at the  time  of  writing
       (5.16), \p{Lu} and \p{Ll} match all letters, regardless of case, when case independence is

       18. 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

       (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 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.

       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.  However, this does not apply to optimized data
       created by the just-in-time compiler.

       (k)  The  alternative   matching   functions   (pcre_dfa_exec(),   pcre16_dfa_exec()   and
       pcre32_dfa_exec(),) match in a different way and are 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.


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


       Last updated: 10 November 2013
       Copyright (c) 1997-2013 University of Cambridge.