pcrecompat(3) — Linux manual page


PCRECOMPAT(3)           Library Functions Manual           PCRECOMPAT(3)

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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 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 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

       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

       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

           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

       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 specified.

       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 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

       (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

       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.

AUTHOR         top

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

REVISION         top

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

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.45.tar.gz fetched from
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PCRE 8.34                   10 November 2013               PCRECOMPAT(3)