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diff --git a/doc/pcre2compat.3 b/doc/pcre2compat.3 index 8333d3e3..8313e03a 100644 --- a/doc/pcre2compat.3 +++ b/doc/pcre2compat.3 @@ -1,13 +1,13 @@ -.TH PCRE2COMPAT 3 "08 December 2021" "PCRE2 10.40" +.TH PCRE2COMPAT 3 "30 November 2023" "PCRE2 10.43" .SH NAME PCRE2 - Perl-compatible regular expressions (revised API) .SH "DIFFERENCES BETWEEN PCRE2 AND PERL" .rs .sp -This document describes some of the differences in the ways that PCRE2 and Perl -handle regular expressions. The differences described here are with respect to -Perl version 5.34.0, but as both Perl and PCRE2 are continually changing, the -information may at times be out of date. +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+respect to Perl version 5.38.0, but as both Perl and PCRE2 are continually +changing, the information may at times be out of date. .P 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 @@ -32,12 +32,16 @@ assertion just once). Perl allows some repeat quantifiers on other assertions, for example, \eb* , but these do not seem to have any use. PCRE2 does not allow any kind of quantifier on non-lookaround assertions. .P -4. Capture groups that occur inside negative lookaround assertions are counted, +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. +.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 assertion is a condition that has a matching branch (that is, the condition is false). Perl may set such capture groups in other circumstances. .P -5. The following Perl escape sequences are not supported: \eF, \el, \eL, \eu, +6. The following Perl escape sequences are not supported: \eF, \el, \eL, \eu, \eU, and \eN when followed by a character name. \eN on its own, matching a non-newline character, and \eN{U+dd..}, matching a Unicode code point, are supported. The escapes that modify the case of following letters are @@ -47,12 +51,13 @@ generated by default. However, if either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \eU and \eu are interpreted as ECMAScript interprets them. .P -6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is +7. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is built with Unicode support (the default). The properties that can be tested with \ep and \eP are limited to the general category properties such as Lu and -Nd, script names such as Greek or Han, Bidi_Class, Bidi_Control, and the -derived properties Any and LC (synonym L&). Both PCRE2 and Perl support the Cs -(surrogate) property, but in PCRE2 its use is limited. See the +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 .\" HREF \fBpcre2pattern\fP .\" @@ -60,7 +65,7 @@ documentation for details. The long synonyms for property names that Perl supports (such as \ep{Letter}) are not supported by PCRE2, nor is it permitted to prefix any of these properties with "Is". .P -7. PCRE2 supports the \eQ...\eE escape for quoting substrings. Characters +8. PCRE2 supports the \eQ...\eE 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 @@ -82,7 +87,7 @@ following examples: The \eQ...\eE sequence is recognized both inside and outside character classes by both PCRE2 and Perl. .P -8. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code}) +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 .\" HREF @@ -90,11 +95,11 @@ external function to be called during pattern matching. See the .\" documentation for details. .P -9. 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. +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. .P -10. In PCRE2, if any of the backtracking control verbs are used in a group that +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 @@ -102,18 +107,18 @@ 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. .P -11. If a pattern contains more than one backtracking control verb, the first +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. .P -12. There are some differences that are concerned with the settings of captured +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". .P -13. PCRE2's handling of duplicate capture group numbers and names is not as +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 @@ -122,81 +127,75 @@ 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. .P -14. Perl used to recognize comments in some places that PCRE2 does not, for +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. .P -15. Perl, when in warning mode, gives warnings for character classes such as +16. Perl, when in warning mode, gives warnings for character classes such as [A-\ed] 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. .P -16. In PCRE2, the upper/lower case character properties Lu and Ll are not +17. In PCRE2, the upper/lower case character properties Lu and Ll are not affected when case-independent matching is specified. For example, \ep{Lu} always matches an upper case letter. I think Perl has changed in this respect; -in the release at the time of writing (5.34), \ep{Lu} and \ep{Ll} match all +in the release at the time of writing (5.38), \ep{Lu} and \ep{Ll} match all letters, regardless of case, when case independence is specified. .P -17. From release 5.32.0, Perl locks out the use of \eK in lookaround +18. From release 5.32.0, Perl locks out the use of \eK 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, \eK is acted on when it occurs in positive assertions, but is ignored in negative assertions. .P -18. PCRE2 provides some extensions to the Perl regular expression facilities. +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 5.34: -.sp -(a) Although lookbehind assertions in PCRE2 must match fixed length strings, -each alternative toplevel branch of a lookbehind assertion can match a -different length of string. Perl used to require them all to have the same -length, but the latest version has some variable length support. +list is with respect to Perl 5.38: .sp -(b) From PCRE2 10.23, backreferences to groups of fixed length are supported -in lookbehinds, provided that there is no possibility of referencing a -non-unique number or name. Perl does not support backreferences in lookbehinds. -.sp -(c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ +(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. .sp -(d) A backslash followed by a letter with no special meaning is faulted. (Perl +(b) A backslash followed by a letter with no special meaning is faulted. (Perl can be made to issue a warning.) .sp -(e) If PCRE2_UNGREEDY is set, the greediness of the repetition quantifiers is +(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. .sp -(f) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried +(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. .sp -(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART +(e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents. .sp -(h) The \eR escape sequence can be restricted to match only CR, LF, or CRLF +(f) The \eR escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF option. .sp -(i) The callout facility is PCRE2-specific. Perl supports codeblocks and +(g) The callout facility is PCRE2-specific. Perl supports codeblocks and variable interpolation, but not general hooks on every match. .sp -(j) The partial matching facility is PCRE2-specific. +(h) The partial matching facility is PCRE2-specific. .sp -(k) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a +(i) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a different way and is not Perl-compatible. .sp -(l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at +(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. .sp -(m) PCRE2 supports non-atomic positive lookaround assertions. This is an +(k) PCRE2 supports non-atomic positive lookaround assertions. This is an extension to the lookaround facilities. The default, Perl-compatible lookarounds are atomic. -.P -19. The Perl /a modifier restricts /d numbers to pure ascii, and the /aa -modifier restricts /i case-insensitive matching to pure ascii, ignoring Unicode -rules. This separation cannot be represented with PCRE2_UCP. +.sp +(l) There are three syntactical items in patterns that can refer to a capturing +group by number: back references such as \eg{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. .P 20. Perl has different limits than PCRE2. See the .\" HREF @@ -206,6 +205,16 @@ 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. +.P +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. +.P +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)|\ez)/, which matches a sequence of any number of repeated +"abcd" substrings at the end of the subject. . . .SH AUTHOR @@ -222,6 +231,6 @@ Cambridge, England. .rs .sp .nf -Last updated: 08 December 2021 -Copyright (c) 1997-2021 University of Cambridge. +Last updated: 30 November 2023 +Copyright (c) 1997-2023 University of Cambridge. .fi |