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<html><head><title>pcre2compat specification</title></head><body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"><h1>pcre2compat man page</h1><p>Return to the <a href="index.html">PCRE2 index page</a>.</p><p>This page is part of the PCRE2 HTML documentation. It was generatedautomatically from the original man page. If there is any nonsense in it,please consult the man page, in case the conversion went wrong.<br><br><b>DIFFERENCES BETWEEN PCRE2 AND PERL</b><br><P>This document describes some of the known differences in the ways that PCRE2and Perl handle regular expressions. The differences described here are withrespect to Perl version 5.38.0, but as both Perl and PCRE2 are continuallychanging, the information may at times be out of date.</P><P>1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not set, thebehaviour of the '.' metacharacter differs from Perl. In PCRE2, '.' matches thenext 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 usingEBCDIC. In Perl, '.' appears never to match LF, even when 0x0A is not a newlineindicator.</P><P>2. PCRE2 has only a subset of Perl's Unicode support. Details of what it doeshave are given in the<a href="pcre2unicode.html"><b>pcre2unicode</b></a>page.</P><P>3. Like Perl, PCRE2 allows repeat quantifiers on parenthesized assertions, butthey do not mean what you might think. For example, (?!a){3} does not assertthat the next three characters are not "a". It just asserts that the nextcharacter is not "a" three times (in principle; PCRE2 optimizes this to run theassertion 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 allowany kind of quantifier on non-lookaround assertions.</P><P>4. If a braced quantifier such as {1,2} appears where there is nothing torepeat (for example, at the start of a branch), PCRE2 raises an error whereasPerl treats the quantifier characters as literal.</P><P>5. Capture groups that occur inside negative lookaround assertions are counted,but their entries in the offsets vector are set only when a negative assertionis a condition that has a matching branch (that is, the condition is false).Perl may set such capture groups in other circumstances.</P><P>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 anon-newline character, and \N{U+dd..}, matching a Unicode code point, aresupported. The escapes that modify the case of following letters areimplemented by Perl's general string-handling and are not part of its patternmatching engine. If any of these are encountered by PCRE2, an error isgenerated by default. However, if either of the PCRE2_ALT_BSUX orPCRE2_EXTRA_ALT_BSUX options is set, \U and \u are interpreted as ECMAScriptinterprets them.</P><P>7. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 isbuilt with Unicode support (the default). The properties that can be testedwith \p and \P are limited to the general category properties such as Lu andNd, the derived properties Any and LC (synonym L&), script names such as Greekor Han, Bidi_Class, Bidi_Control, and a few binary properties. Both PCRE2 andPerl support the Cs (surrogate) property, but in PCRE2 its use is limited. Seethe<a href="pcre2pattern.html"><b>pcre2pattern</b></a>documentation for details. The long synonyms for property names that Perlsupports (such as \p{Letter}) are not supported by PCRE2, nor is it permittedto prefix any of these properties with "Is".</P><P>8. PCRE2 supports the \Q...\E escape for quoting substrings. Charactersin between are treated as literals. However, this is slightly different fromPerl in that $ and @ are also handled as literals inside the quotes. In Perl,they cause variable interpolation (PCRE2 does not have variables). Also, Perldoes "double-quotish backslash interpolation" on any backslashes between \Qand \E which, its documentation says, "may lead to confusing results". PCRE2treats a backslash between \Q and \E just like any other character. Note thefollowing examples:<pre> 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$xyz \QA\B\E A\B A\B \Q\\E \ \\E</pre>The \Q...\E sequence is recognized both inside and outside character classesby both PCRE2 and Perl.</P><P>9. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})constructions. However, PCRE2 does have a "callout" feature, which allows anexternal function to be called during pattern matching. See the<a href="pcre2callout.html"><b>pcre2callout</b></a>documentation for details.</P><P>10. Subroutine calls (whether recursive or not) were treated as atomic groupsup to PCRE2 release 10.23, but from release 10.30 this changed, andbacktracking into subroutine calls is now supported, as in Perl.</P><P>11. In PCRE2, if any of the backtracking control verbs are used in a group thatis called as a subroutine (whether or not recursively), their effect isconfined to that group; it does not extend to the surrounding pattern. This isnot always the case in Perl. In particular, if (*THEN) is present in a groupthat is called as a subroutine, its action is limited to that group, even ifthe group does not contain any | characters. Note that such groups areprocessed as anchored at the point where they are tested.</P><P>12. If a pattern contains more than one backtracking control verb, the firstone that is backtracked onto acts. For example, in the patternA(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in Ctriggers (*PRUNE). Perl's behaviour is more complex; in many cases it is thesame as PCRE2, but there are cases where it differs.</P><P>13. There are some differences that are concerned with the settings of capturedstrings when part of a pattern is repeated. For example, matching "aba" againstthe pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to"b".</P><P>14. PCRE2's handling of duplicate capture group numbers and names is not asgeneral as Perl's. This is a consequence of the fact the PCRE2 works internallyjust with numbers, using an external table to translate between numbers andnames. In particular, a pattern such as (?|(?<a>A)|(?<b>B)), where the twocapture groups have the same number but different names, is not supported, andcauses an error at compile time. If it were allowed, it would not be possibleto distinguish which group matched, because both names map to capture groupnumber 1. To avoid this confusing situation, an error is given at compile time.</P><P>15. Perl used to recognize comments in some places that PCRE2 does not, forexample, between the ( and ? at the start of a group. If the /x modifier isset, Perl allowed white space between ( and ? though the latest Perls give anerror (for a while it was just deprecated). There may still be some cases wherePerl behaves differently.</P><P>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 nowarning features, so it gives an error in these cases because they are almostcertainly user mistakes.</P><P>17. In PCRE2, the upper/lower case character properties Lu and Ll are notaffected 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 (5.38), \p{Lu} and \p{Ll} match allletters, regardless of case, when case independence is specified.</P><P>18. From release 5.32.0, Perl locks out the use of \K in lookaroundassertions. From release 10.38 PCRE2 does the same by default. However, thereis 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 innegative assertions.</P><P>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, someof which (such as named parentheses) were in PCRE2 for some time before. Thislist is with respect to Perl 5.38:<br><br>(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.<br><br>(b) A backslash followed by a letter with no special meaning is faulted. (Perlcan be made to issue a warning.)<br><br>(c) If PCRE2_UNGREEDY is set, the greediness of the repetition quantifiers isinverted, that is, by default they are not greedy, but if followed by aquestion mark they are.<br><br>(d) PCRE2_ANCHORED can be used at matching time to force a pattern to be triedonly at the first matching position in the subject string.<br><br>(e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTARToptions have no Perl equivalents.<br><br>(f) The \R escape sequence can be restricted to match only CR, LF, or CRLFby the PCRE2_BSR_ANYCRLF option.<br><br>(g) The callout facility is PCRE2-specific. Perl supports codeblocks andvariable interpolation, but not general hooks on every match.<br><br>(h) The partial matching facility is PCRE2-specific.<br><br>(i) The alternative matching function (<b>pcre2_dfa_match()</b> matches in adifferent way and is not Perl-compatible.<br><br>(j) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) atthe start of a pattern. These set overall options that cannot be changed withinthe pattern.<br><br>(k) PCRE2 supports non-atomic positive lookaround assertions. This is anextension to the lookaround facilities. The default, Perl-compatiblelookarounds are atomic.<br><br>(l) There are three syntactical items in patterns that can refer to a capturinggroup by number: back references such as \g{2}, subroutine calls such as (?3),and condition references such as (?(4)...). PCRE2 supports relative groupnumbers such as +2 and -4 in all three cases. Perl supports both plus and minusfor subroutine calls, but only minus for back references, and no relativenumbering at all for conditions.</P><P>20. Perl has different limits than PCRE2. See the<a href="pcre2limit.html"><b>pcre2limit</b></a>documentation for details. Perl went with 5.10 from recursion to iterationkeeping the intermediate matches on the heap, which is ~10% slower but does notfall into any stack-overflow limit. PCRE2 made a similar change at release10.30, and also has many build-time and run-time customizable limits.</P><P>21. Unlike Perl, PCRE2 doesn't have character set modifiers and specially no wayto set characters by context just like Perl's "/d". A regular expression usingPCRE2_UTF and PCRE2_UCP will use similar rules to Perl's "/u"; something closerto "/a" could be selected by adding other PCRE2_EXTRA_ASCII* options on top.</P><P>22. Some recursive patterns that Perl diagnoses as infinite recursions can behandled 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.</P><br><b>AUTHOR</b><br><P>Philip Hazel<br>Retired from University Computing Service<br>Cambridge, England.<br></P><br><b>REVISION</b><br><P>Last updated: 30 November 2023<br>Copyright © 1997-2023 University of Cambridge.<br><p>Return to the <a href="index.html">PCRE2 index page</a>.</p>
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