1PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3)
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6 PCRE2 - Perl-compatible regular expressions (revised API)
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9 This document describes some of the differences in the ways that PCRE2
11 and Perl handle regular expressions. The differences described here are
12 with respect to Perl version 5.32.0, but as both Perl and PCRE2 are
13 continually changing, the information may at times be out of date.
14 1. PCRE2 has only a subset of Perl's Unicode support. Details of what
16 it does have are given in the pcre2unicode page.
17 2. Like Perl, PCRE2 allows repeat quantifiers on parenthesized asser‐
19 tions, but they do not mean what you might think. For example, (?!a){3}
20 does not assert that the next three characters are not "a". It just
21 asserts that the next character is not "a" three times (in principle;
22 PCRE2 optimizes this to run the assertion just once). Perl allows some
23 repeat quantifiers on other assertions, for example, \b* (but not
24 \b{3}, though oddly it does allow ^{3}), but these do not seem to have
25 any use. PCRE2 does not allow any kind of quantifier on non-lookaround
26 assertions.
27 3. Capture groups that occur inside negative lookaround assertions are
29 counted, but their entries in the offsets vector are set only when a
30 negative assertion is a condition that has a matching branch (that is,
31 the condition is false). Perl may set such capture groups in other
32 circumstances.
33 4. The following Perl escape sequences are not supported: \F, \l, \L,
35 \u, \U, and \N when followed by a character name. \N on its own, match‐
36 ing a non-newline character, and \N{U+dd..}, matching a Unicode code
37 point, are supported. The escapes that modify the case of following
38 letters are implemented by Perl's general string-handling and are not
39 part of its pattern matching engine. If any of these are encountered by
40 PCRE2, an error is generated by default. However, if either of the
41 PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are
42 interpreted as ECMAScript interprets them.
43 5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
45 is built with Unicode support (the default). The properties that can be
46 tested with \p and \P are limited to the general category properties
47 such as Lu and Nd, script names such as Greek or Han, and the derived
48 properties Any and L&. Both PCRE2 and Perl support the Cs (surrogate)
49 property, but in PCRE2 its use is limited. See the pcre2pattern docu‐
50 mentation for details. The long synonyms for property names that Perl
51 supports (such as \p{Letter}) are not supported by PCRE2, nor is it
52 permitted to prefix any of these properties with "Is".
53 6. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
55 in between are treated as literals. However, this is slightly different
56 from Perl in that $ and @ are also handled as literals inside the
57 quotes. In Perl, they cause variable interpolation (but of course PCRE2
58 does not have variables). Also, Perl does "double-quotish backslash
59 interpolation" on any backslashes between \Q and \E which, its documen‐
60 tation says, "may lead to confusing results". PCRE2 treats a backslash
61 between \Q and \E just like any other character. Note the following
62 examples:
63 Pattern PCRE2 matches Perl matches
65 \Qabc$xyz\E abc$xyz abc followed by the
67 contents of $xyz
68 \Qabc\$xyz\E abc\$xyz abc\$xyz
69 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
70 \QA\B\E A\B A\B
71 \Q\\E \ \\E
72 The \Q...\E sequence is recognized both inside and outside character
74 classes by both PCRE2 and Perl.
75 7. Fairly obviously, PCRE2 does not support the (?{code}) and
77 (??{code}) constructions. However, PCRE2 does have a "callout" feature,
78 which allows an external function to be called during pattern matching.
79 See the pcre2callout documentation for details.
80 8. Subroutine calls (whether recursive or not) were treated as atomic
82 groups up to PCRE2 release 10.23, but from release 10.30 this changed,
83 and backtracking into subroutine calls is now supported, as in Perl.
84 9. In PCRE2, if any of the backtracking control verbs are used in a
86 group that is called as a subroutine (whether or not recursively),
87 their effect is confined to that group; it does not extend to the sur‐
88 rounding pattern. This is not always the case in Perl. In particular,
89 if (*THEN) is present in a group that is called as a subroutine, its
90 action is limited to that group, even if the group does not contain any
91 | characters. Note that such groups are processed as anchored at the
92 point where they are tested.
93 10. If a pattern contains more than one backtracking control verb, the
95 first one that is backtracked onto acts. For example, in the pattern
96 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
97 in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
98 it is the same as PCRE2, but there are cases where it differs.
99 11. There are some differences that are concerned with the settings of
101 captured strings when part of a pattern is repeated. For example,
102 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
103 unset, but in PCRE2 it is set to "b".
104 12. PCRE2's handling of duplicate capture group numbers and names is
106 not as general as Perl's. This is a consequence of the fact the PCRE2
107 works internally just with numbers, using an external table to trans‐
108 late between numbers and names. In particular, a pattern such as
109 (?|(?<a>A)|(?<b>B)), where the two capture groups have the same number
110 but different names, is not supported, and causes an error at compile
111 time. If it were allowed, it would not be possible to distinguish which
112 group matched, because both names map to capture group number 1. To
113 avoid this confusing situation, an error is given at compile time.
114 13. Perl used to recognize comments in some places that PCRE2 does not,
116 for example, between the ( and ? at the start of a group. If the /x
117 modifier is set, Perl allowed white space between ( and ? though the
118 latest Perls give an error (for a while it was just deprecated). There
119 may still be some cases where Perl behaves differently.
120 14. Perl, when in warning mode, gives warnings for character classes
122 such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter‐
123 als. PCRE2 has no warning features, so it gives an error in these cases
124 because they are almost certainly user mistakes.
125 15. In PCRE2, the upper/lower case character properties Lu and Ll are
127 not affected when case-independent matching is specified. For example,
128 \p{Lu} always matches an upper case letter. I think Perl has changed in
129 this respect; in the release at the time of writing (5.32), \p{Lu} and
130 \p{Ll} match all letters, regardless of case, when case independence is
131 specified.
132 16. From release 5.32.0, Perl locks out the use of \K in lookaround
134 assertions. In PCRE2, \K is acted on when it occurs in positive asser‐
135 tions, but is ignored in negative assertions.
136 17. PCRE2 provides some extensions to the Perl regular expression
138 facilities. Perl 5.10 included new features that were not in earlier
139 versions of Perl, some of which (such as named parentheses) were in
140 PCRE2 for some time before. This list is with respect to Perl 5.32:
141 (a) Although lookbehind assertions in PCRE2 must match fixed length
143 strings, each alternative toplevel branch of a lookbehind assertion can
144 match a different length of string. Perl requires them all to have the
145 same length.
146 (b) From PCRE2 10.23, backreferences to groups of fixed length are sup‐
148 ported in lookbehinds, provided that there is no possibility of refer‐
149 encing a non-unique number or name. Perl does not support backrefer‐
150 ences in lookbehinds.
151 (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
153 $ meta-character matches only at the very end of the string.
154 (d) A backslash followed by a letter with no special meaning is
156 faulted. (Perl can be made to issue a warning.)
157 (e) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti‐
159 fiers is inverted, that is, by default they are not greedy, but if fol‐
160 lowed by a question mark they are.
161 (f) PCRE2_ANCHORED can be used at matching time to force a pattern to
163 be tried only at the first matching position in the subject string.
164 (g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and
166 PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
167 (h) The \R escape sequence can be restricted to match only CR, LF, or
169 CRLF by the PCRE2_BSR_ANYCRLF option.
170 (i) The callout facility is PCRE2-specific. Perl supports codeblocks
172 and variable interpolation, but not general hooks on every match.
173 (j) The partial matching facility is PCRE2-specific.
175 (k) The alternative matching function (pcre2_dfa_match() matches in a
177 different way and is not Perl-compatible.
178 (l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT)
180 at the start of a pattern. These set overall options that cannot be
181 changed within the pattern.
182 (m) PCRE2 supports non-atomic positive lookaround assertions. This is
184 an extension to the lookaround facilities. The default, Perl-compatible
185 lookarounds are atomic.
186 18. The Perl /a modifier restricts /d numbers to pure ascii, and the
188 /aa modifier restricts /i case-insensitive matching to pure ascii,
189 ignoring Unicode rules. This separation cannot be represented with
190 PCRE2_UCP.
191 19. Perl has different limits than PCRE2. See the pcre2limit documenta‐
193 tion for details. Perl went with 5.10 from recursion to iteration keep‐
194 ing the intermediate matches on the heap, which is ~10% slower but does
195 not fall into any stack-overflow limit. PCRE2 made a similar change at
196 release 10.30, and also has many build-time and run-time customizable
197 limits.
198
200 Philip Hazel
202 University Computing Service
203 Cambridge, England.
204
206 Last updated: 06 October 2020
208 Copyright (c) 1997-2019 University of Cambridge.
209PCRE2 10.36 06 October 2020 PCRE2COMPAT(3)