PCRECOMPAT(3)                                                    PCRECOMPAT(3)



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 versions 5.10 and above.

       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
       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 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'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.

       12. Perl recognizes comments in some places that PCRE doesn't, for
       example, between the ( and ? at the start of a subpattern.

       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
       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.

       (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
       and 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.

       (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
       pattern.

AUTHOR

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

REVISION

       Last updated: 31 October 2010
       Copyright (c) 1997-2010 University of Cambridge.



                                                                 PCRECOMPAT(3)
