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 the differences in the ways that PCRE2 and Perl
11 handle regular expressions. The differences described here are with
12 respect to Perl versions 5.26, but as both Perl and PCRE2 are continu‐
13 ally changing, the information may sometimes be out of date.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}), but these do not seem to have any use.
25 3. Capturing subpatterns that occur inside negative lookaround asser‐
27 tions are counted, but their entries in the offsets vector are set only
28 when a negative assertion is a condition that has a matching branch
29 (that is, the condition is false).
30 4. The following Perl escape sequences are not supported: \F, \l, \L,
32 \u, \U, and \N when followed by a character name. \N on its own, match‐
33 ing a non-newline character, and \N{U+dd..}, matching a Unicode code
34 point, are supported. The escapes that modify the case of following
35 letters are implemented by Perl's general string-handling and are not
36 part of its pattern matching engine. If any of these are encountered by
37 PCRE2, an error is generated by default. However, if the PCRE2_ALT_BSUX
38 option is set, \U and \u are interpreted as ECMAScript interprets them.
39 5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
41 is built with Unicode support (the default). The properties that can be
42 tested with \p and \P are limited to the general category properties
43 such as Lu and Nd, script names such as Greek or Han, and the derived
44 properties Any and L&. PCRE2 does support the Cs (surrogate) property,
45 which Perl does not; the Perl documentation says "Because Perl hides
46 the need for the user to understand the internal representation of Uni‐
47 code characters, there is no need to implement the somewhat messy con‐
48 cept of surrogates."
49 6. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
51 in between are treated as literals. However, this is slightly different
52 from Perl in that $ and @ are also handled as literals inside the
53 quotes. In Perl, they cause variable interpolation (but of course PCRE2
54 does not have variables). Also, Perl does "double-quotish backslash
55 interpolation" on any backslashes between \Q and \E which, its documen‐
56 tation says, "may lead to confusing results". PCRE2 treats a backslash
57 between \Q and \E just like any other character. Note the following
58 examples:
59 Pattern PCRE2 matches Perl matches
61 \Qabc$xyz\E abc$xyz abc followed by the
63 contents of $xyz
64 \Qabc\$xyz\E abc\$xyz abc\$xyz
65 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
66 \QA\B\E A\B A\B
67 \Q\\E \ \\E
68 The \Q...\E sequence is recognized both inside and outside character
70 classes.
71 7. Fairly obviously, PCRE2 does not support the (?{code}) and
73 (??{code}) constructions. However, PCRE2 does have a "callout" feature,
74 which allows an external function to be called during pattern matching.
75 See the pcre2callout documentation for details.
76 8. Subroutine calls (whether recursive or not) were treated as atomic
78 groups up to PCRE2 release 10.23, but from release 10.30 this changed,
79 and backtracking into subroutine calls is now supported, as in Perl.
80 9. If any of the backtracking control verbs are used in a subpattern
82 that is called as a subroutine (whether or not recursively), their
83 effect is confined to that subpattern; it does not extend to the sur‐
84 rounding pattern. This is not always the case in Perl. In particular,
85 if (*THEN) is present in a group that is called as a subroutine, its
86 action is limited to that group, even if the group does not contain any
87 | characters. Note that such subpatterns are processed as anchored at
88 the point where they are tested.
89 10. If a pattern contains more than one backtracking control verb, the
91 first one that is backtracked onto acts. For example, in the pattern
92 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
93 in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
94 it is the same as PCRE2, but there are cases where it differs.
95 11. Most backtracking verbs in assertions have their normal actions.
97 They are not confined to the assertion.
98 12. There are some differences that are concerned with the settings of
100 captured strings when part of a pattern is repeated. For example,
101 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
102 unset, but in PCRE2 it is set to "b".
103 13. PCRE2's handling of duplicate subpattern numbers and duplicate sub‐
105 pattern names is not as general as Perl's. This is a consequence of the
106 fact the PCRE2 works internally just with numbers, using an external
107 table to translate between numbers and names. In particular, a pattern
108 such as (?|(?<a>A)|(?<b>B), where the two capturing parentheses have
109 the same number but different names, is not supported, and causes an
110 error at compile time. If it were allowed, it would not be possible to
111 distinguish which parentheses matched, because both names map to cap‐
112 turing subpattern number 1. To avoid this confusing situation, an error
113 is given at compile time.
114 14. Perl used to recognize comments in some places that PCRE2 does not,
116 for example, between the ( and ? at the start of a subpattern. If the
117 /x 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 15. 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 16. 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 (5.24), \p{Lu} and
130 \p{Ll} match all letters, regardless of case, when case independence is
131 specified.
132 17. PCRE2 provides some extensions to the Perl regular expression
134 facilities. Perl 5.10 includes new features that are not in earlier
135 versions of Perl, some of which (such as named parentheses) were in
136 PCRE2 for some time before. This list is with respect to Perl 5.26:
137 (a) Although lookbehind assertions in PCRE2 must match fixed length
139 strings, each alternative branch of a lookbehind assertion can match a
140 different length of string. Perl requires them all to have the same
141 length.
142 (b) From PCRE2 10.23, backreferences to groups of fixed length are sup‐
144 ported in lookbehinds, provided that there is no possibility of refer‐
145 encing a non-unique number or name. Perl does not support backrefer‐
146 ences in lookbehinds.
147 (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
149 $ meta-character matches only at the very end of the string.
150 (d) A backslash followed by a letter with no special meaning is
152 faulted. (Perl can be made to issue a warning.)
153 (e) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti‐
155 fiers is inverted, that is, by default they are not greedy, but if fol‐
156 lowed by a question mark they are.
157 (f) PCRE2_ANCHORED can be used at matching time to force a pattern to
159 be tried only at the first matching position in the subject string.
160 (g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and
162 PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
163 (h) The \R escape sequence can be restricted to match only CR, LF, or
165 CRLF by the PCRE2_BSR_ANYCRLF option.
166 (i) The callout facility is PCRE2-specific. Perl supports codeblocks
168 and variable interpolation, but not general hooks on every match.
169 (j) The partial matching facility is PCRE2-specific.
171 (k) The alternative matching function (pcre2_dfa_match() matches in a
173 different way and is not Perl-compatible.
174 (l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT)
176 at the start of a pattern that set overall options that cannot be
177 changed within the pattern.
178 18. The Perl /a modifier restricts /d numbers to pure ascii, and the
180 /aa modifier restricts /i case-insensitive matching to pure ascii,
181 ignoring Unicode rules. This separation cannot be represented with
182 PCRE2_UCP.
183 19. Perl has different limits than PCRE2. See the pcre2limit documenta‐
185 tion for details. Perl went with 5.10 from recursion to iteration keep‐
186 ing the intermediate matches on the heap, which is ~10% slower but does
187 not fall into any stack-overflow limit. PCRE2 made a similar change at
188 release 10.30, and also has many build-time and run-time customizable
189 limits.
190
192 Philip Hazel
194 University Computing Service
195 Cambridge, England.
196
198 Last updated: 28 July 2018
200 Copyright (c) 1997-2018 University of Cambridge.
201PCRE2 10.32 28 July 2018 PCRE2COMPAT(3)