PCRECOMPAT(3)             OpenBSD Programmer's Manual            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 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 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 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 implements a simpler version of \X than Perl, which changed to
       make \X match what Unicode calls an "extended grapheme cluster". This
       is more complicated than an extended Unicode sequence, which is what
       PCRE matches.

       8. 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.

       9. 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.

       10. 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.

       11. 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.

       12. 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".

       13. 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.

       14. 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 whitespace between ( and ? but PCRE never
       does, even if the PCRE_EXTENDED option is set.

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

       (k) The alternative matching functions (pcre_dfa_exec() and
       pcre16_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.

AUTHOR

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

REVISION

       Last updated: 08 Januray 2012
       Copyright (c) 1997-2012 University of Cambridge.



                                                                 PCRECOMPAT(3)
