# pcre2compat

## NAME



           PCRE2 - Perl-compatible regular expressions (revised API)
    


## DIFFERENCES BETWEEN PCRE2 AND PERL



    
           This document describes some of the known differences in the ways
           that PCRE2 and Perl handle regular expressions. The differences
           described here are with respect to Perl version 5.38.0, but as
           both Perl and PCRE2 are continually changing, the information may
           at times be out of date.
    
           1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not
           set, the behaviour of the '.' metacharacter differs from Perl. In
           PCRE2, '.' matches the next character unless it is the start of a
           newline sequence. This means that, if the newline setting is CR,
           CRLF, or NUL, '.' will match the code point LF (0x0A) in
           ASCII/Unicode environments, and NL (either 0x15 or 0x25) when
           using EBCDIC. In Perl, '.' appears never to match LF, even when
           0x0A is not a newline indicator.
    
           2. PCRE2 has only a subset of Perl's Unicode support. Details of
           what it does have are given in the **pcre2unicode** page.
    
           3. Like Perl, PCRE2 allows repeat quantifiers 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; PCRE2 optimizes this to run the
           assertion just once). Perl allows some repeat quantifiers on other
           assertions, for example, \b* , but these do not seem to have any
           use. PCRE2 does not allow any kind of quantifier on non-lookaround
           assertions.
    
           4. If a braced quantifier such as {1,2} appears where there is
           nothing to repeat (for example, at the start of a branch), PCRE2
           raises an error whereas Perl treats the quantifier characters as
           literal. When a braced quantifier (...){min,max} has min > max,
           Perl treats it as an item which fails to match any portion of the
           subject (as no number of repetitions can meet the condition), and
           additionally issues a warning when in warning mode. PCRE2 has no
           warning features, so it gives an error in this case.
    
           5. Capture groups that occur inside negative lookaround assertions
           are counted, but their entries in the offsets vector are set only
           when a negative assertion is a condition that has a matching
           branch (that is, the condition is false).  Perl may set such
           capture groups in other circumstances.
    
           6. The following Perl escape sequences are not supported: \F, \l,
           \L, \u, \U, and \N when followed by a character name. \N on its
           own, matching a non-newline character, and \N{U+dd..}, matching a
           Unicode code point, are supported. The escapes that modify the
           case of following letters are implemented by Perl's general
           string-handling and are not part of its pattern matching engine.
           If any of these are encountered by PCRE2, an error is generated by
           default. However, if either of the PCRE2_ALT_BSUX or
           PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are interpreted as
           ECMAScript interprets them.
    
           7. The Perl escape sequences \p, \P, and \X are supported only if
           PCRE2 is built with Unicode support (the default). The properties
           that can be tested with \p and \P are limited to the general
           category properties such as Lu and Nd, the derived properties Any
           and Lc (synonym L&), script names such as Greek or Han,
           Bidi_Class, Bidi_Control, and a few binary properties. Both PCRE2
           and Perl support the Cs (surrogate) property, but in PCRE2 its use
           is limited. See the **pcre2pattern** documentation for details. The
           long synonyms for property names that Perl supports (such as
           \p{Letter}) are not supported by PCRE2, nor is it permitted to
           prefix any of these properties with "Is".
    
           8. PCRE2 supports the \Q...\E escape for quoting substrings.
           Characters in between are treated as literals. However, this is
           slightly different from Perl in that $ and @ are also handled as
           literals inside the quotes. In Perl, they cause variable
           interpolation (PCRE2 does not have variables). Also, Perl does
           "double-quotish backslash interpolation" on any backslashes
           between \Q and \E which, its documentation says, "may lead to
           confusing results". PCRE2 treats a backslash between \Q and \E
           just like any other character. Note the following examples:
    
               Pattern            PCRE2 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
               \QA\B\E            A\B               A\B
               \Q\\E              \                 \\E
    
           The \Q...\E sequence is recognized both inside and outside
           character classes by both PCRE2 and Perl. Another difference from
           Perl is that any appearance of \Q or \E inside what might
           otherwise be a quantifier causes PCRE2 not to recognize the
           sequence as a quantifier. Perl recognizes a quantifier if
           (redundantly) either of the numbers is inside \Q...\E, but not if
           the separating comma is. When not recognized as a quantifier a
           sequence such as {\Q1\E,2} is treated as the literal string
           "{1,2}".
    
           9. Fairly obviously, PCRE2 does not support the (?{code}) and
           (??{code}) constructions. However, PCRE2 does have a "callout"
           feature, which allows an external function to be called during
           pattern matching. See the **pcre2callout** documentation for details.
    
           10. Subroutine calls (whether recursive or not) were treated as
           atomic groups up to PCRE2 release 10.23, but from release 10.30
           this changed, and backtracking into subroutine calls is now
           supported, as in Perl.
    
           11. In PCRE2, if any of the backtracking control verbs are used in
           a group that is called as a subroutine (whether or not
           recursively), their effect is confined to that group; 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
           groups are processed as anchored at the point where they are
           tested. PCRE2 also confines all control verbs within atomic
           assertions, again including (*THEN) in assertions with only one
           branch.
    
           12. 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 PCRE2, but there are
           cases where it differs.
    
           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 PCRE2 it is set to "b".
    
           14. PCRE2's handling of duplicate capture group numbers and names
           is not as general as Perl's. This is a consequence of the fact the
           PCRE2 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 capture groups 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 group matched, because both names
           map to capture group number 1. To avoid this confusing situation,
           an error is given at compile time.
    
           15. Perl used to recognize comments in some places that PCRE2 does
           not, for example, between the ( and ? at the start of a group. If
           the /x modifier is set, Perl allowed white space between ( and ?
           though the latest Perls give an error (for a while it was just
           deprecated). There may still be some cases where Perl behaves
           differently.
    
           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. PCRE2 has no warning features, so it gives an
           error in these cases because they are almost certainly user
           mistakes.
    
           17. In PCRE2, until release 10.45, the upper/lower case character
           properties Lu and Ll were not affected when case-independent
           matching was specified. Perl has changed in this respect, and
           PCRE2 has now changed to match. When caseless matching is in
           force, Lu, Ll, and Lt (title case) are all treated as Lc (cased
           letter).
    
           18. From release 5.32.0, Perl locks out the use of \K in
           lookaround assertions. From release 10.38 PCRE2 does the same by
           default. However, there is an option for re-enabling the previous
           behaviour. When this option is set, \K is acted on when it occurs
           in positive assertions, but is ignored in negative assertions.
    
           19. PCRE2 provides some extensions to the Perl regular expression
           facilities.  Perl 5.10 included new features that were not in
           earlier versions of Perl, some of which (such as named
           parentheses) were in PCRE2 for some time before. This list is with
           respect to Perl 5.38:
    
           (a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set,
           the $ meta-character matches only at the very end of the string.
    
           (b) A backslash followed by a letter with no special meaning is
           faulted. (Perl can be made to issue a warning.)
    
           (c) If PCRE2_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.
    
           (d) PCRE2_ANCHORED can be used at matching time to force a pattern
           to be tried only at the first matching position in the subject
           string.
    
           (e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and
           PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
    
           (f) The \R escape sequence can be restricted to match only CR, LF,
           or CRLF by the PCRE2_BSR_ANYCRLF option.
    
           (g) The callout facility is PCRE2-specific. Perl supports
           codeblocks and variable interpolation, but not general hooks on
           every match.
    
           (h) The partial matching facility is PCRE2-specific.
    
           (i) The alternative matching function (**pcre2_dfa_match()**) matches
           in a different way and is not Perl-compatible.
    
           (j) PCRE2 recognizes some special sequences such as (*CR) or
           (*NO_JIT) at the start of a pattern. These set overall options
           that cannot be changed within the pattern.
    
           (k) PCRE2 supports non-atomic positive lookaround assertions. This
           is an extension to the lookaround facilities. The default, Perl-
           compatible lookarounds are atomic.
    
           (l) There are three syntactical items in patterns that can refer
           to a capturing group by number: back references such as \g{2},
           subroutine calls such as (?3), and condition references such as
           (?(4)...). PCRE2 supports relative group numbers such as +2 and -4
           in all three cases. Perl supports both plus and minus for
           subroutine calls, but only minus for back references, and no
           relative numbering at all for conditions.
    
           (m) The scan substring assertion (syntax (*scs:(n)...)) is a PCRE2
           extension that is not available in Perl.
    
           20. Perl has different limits than PCRE2. See the **pcre2limits**
           documentation for details. Perl went with 5.10 from recursion to
           iteration keeping the intermediate matches on the heap, which is
           ~10% slower but does not fall into any stack-overflow limit. PCRE2
           made a similar change at release 10.30, and also has many build-
           time and run-time customizable limits.
    
           21. Unlike Perl, PCRE2 doesn't have character set modifiers and
           specially no way to set characters by context just like Perl's
           "/d". A regular expression using PCRE2_UTF and PCRE2_UCP will use
           similar rules to Perl's "/u"; something closer to "/a" could be
           selected by adding other PCRE2_EXTRA_ASCII* options on top.
    
           22. Some recursive patterns that Perl diagnoses as infinite
           recursions can be handled by PCRE2, either by the interpreter or
           the JIT. An example is /(?:|(?0)abcd)(?(R)|\z)/, which matches a
           sequence of any number of repeated "abcd" substrings at the end of
           the subject.
    
           23. Both PCRE2 and Perl error when \x{ escapes are invalid, but
           Perl tries to recover and prints a warning if the problem was that
           an invalid hexadecimal digit was found. Since PCRE2 doesn't have
           warnings it returns an error instead.  Additionally, Perl accepts
           \x{} and generates NUL unlike PCRE2.
    
           24. From release 10.45, PCRE2 gives an error if \x is not followed
           by a hexadecimal digit or a curly bracket. It used to interpret
           this as the NUL character. Perl still generates NUL, but warns
           when in warning mode in most cases.
    


## REVISION



    
           Last updated: 02 October 2024
           Copyright (c) 1997-2024 University of Cambridge.
    


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