PCRECOMPAT(3)              Library Functions Manual              PCRECOMPAT(3)

       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 re-
       spect to Perl versions 5.10 and above.

       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 asser-
       tions 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 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 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_COM-
       PAT  option  is set, \U and \u are interpreted as JavaScript interprets

       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&. 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 representa-
       tion  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. 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

       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  pat-
       tern matching. See the pcrecallout documentation for details.

       9.  Subpatterns  that  are called as subroutines (whether or not recur-
       sively) are always treated as atomic  groups  in  PCRE.  This  is  like
       Python,  but  unlike Perl.  Captured values that are set outside a sub-
       routine 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 ef-
       fect is confined to that subpattern; it does not  extend  to  the  sur-
       rounding  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  un-
       set, but in PCRE it is set to "b".

       14.  PCRE's handling of duplicate subpattern numbers and duplicate sub-
       pattern 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 ta-
       ble 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  cap-
       turing 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 ex-
       ample, between the ( and ? at the start of a subpattern. If the /x mod-
       ifier 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 liter-
       als. 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 facil-
       ities.   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

       (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 quietly
       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.

       and  PCRE_NO_AUTO_CAPTURE  options for pcre_exec() have no Perl equiva-

       (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


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


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

PCRE 8.34                      10 November 2013                  PCRECOMPAT(3)
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