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 5.10 and above.
13 1. PCRE2 has only a subset of Perl's Unicode support. Details of what
15 it does have are given in the pcre2unicode page.
16 2. PCRE2 allows repeat quantifiers only on parenthesized assertions,
18 but they do not mean what you might think. For example, (?!a){3} does
19 not assert that the next three characters are not "a". It just asserts
20 that the next character is not "a" three times (in principle: PCRE2
21 optimizes this to run the assertion just once). Perl allows repeat
22 quantifiers on other assertions such as \b, but these do not seem to
23 have any use.
24 3. Capturing subpatterns that occur inside negative lookahead asser‐
26 tions are counted, but their entries in the offsets vector are never
27 set. Perl sometimes (but not always) sets its numerical variables from
28 inside negative assertions.
29 4. The following Perl escape sequences are not supported: \l, \u, \L,
31 \U, and \N when followed by a character name or Unicode value. (\N on
32 its own, matching a non-newline character, is supported.) In fact these
33 are implemented by Perl's general string-handling and are not part of
34 its pattern matching engine. If any of these are encountered by PCRE2,
35 an error is generated by default. However, if the PCRE2_ALT_BSUX option
36 is set, \U and \u are interpreted as ECMAScript interprets them.
37 5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
39 is built with Unicode support. The properties that can be tested with
40 \p and \P are limited to the general category properties such as Lu and
41 Nd, script names such as Greek or Han, and the derived properties Any
42 and L&. PCRE2 does support the Cs (surrogate) property, which Perl does
43 not; the Perl documentation says "Because Perl hides the need for the
44 user to understand the internal representation of Unicode characters,
45 there is no need to implement the somewhat messy concept of surro‐
46 gates."
47 6. PCRE2 does support the \Q...\E escape for quoting substrings. Char‐
49 acters in between are treated as literals. This is slightly different
50 from Perl in that $ and @ are also handled as literals inside the
51 quotes. In Perl, they cause variable interpolation (but of course PCRE2
52 does not have variables). Note the following examples:
53 Pattern PCRE2 matches Perl matches
55 \Qabc$xyz\E abc$xyz abc followed by the
57 contents of $xyz
58 \Qabc\$xyz\E abc\$xyz abc\$xyz
59 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
60 The \Q...\E sequence is recognized both inside and outside character
62 classes.
63 7. Fairly obviously, PCRE2 does not support the (?{code}) and
65 (??{code}) constructions. However, there is support for recursive pat‐
66 terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also,
67 the PCRE2 "callout" feature allows an external function to be called
68 during pattern matching. See the pcre2callout documentation for
69 details.
70 8. Subroutine calls (whether recursive or not) are treated as atomic
72 groups. Atomic recursion is like Python, but unlike Perl. Captured
73 values that are set outside a subroutine call can be referenced from
74 inside in PCRE2, but not in Perl. There is a discussion that explains
75 these differences in more detail in the section on recursion differ‐
76 ences from Perl in the pcre2pattern page.
77 9. If any of the backtracking control verbs are used in a subpattern
79 that is called as a subroutine (whether or not recursively), their
80 effect is confined to that subpattern; it does not extend to the sur‐
81 rounding pattern. This is not always the case in Perl. In particular,
82 if (*THEN) is present in a group that is called as a subroutine, its
83 action is limited to that group, even if the group does not contain any
84 | characters. Note that such subpatterns are processed as anchored at
85 the point where they are tested.
86 10. If a pattern contains more than one backtracking control verb, the
88 first one that is backtracked onto acts. For example, in the pattern
89 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
90 in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
91 it is the same as PCRE2, but there are cases where it differs.
92 11. Most backtracking verbs in assertions have their normal actions.
94 They are not confined to the assertion.
95 12. There are some differences that are concerned with the settings of
97 captured strings when part of a pattern is repeated. For example,
98 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
99 unset, but in PCRE2 it is set to "b".
100 13. PCRE2's handling of duplicate subpattern numbers and duplicate sub‐
102 pattern names is not as general as Perl's. This is a consequence of the
103 fact the PCRE2 works internally just with numbers, using an external
104 table to translate between numbers and names. In particular, a pattern
105 such as (?|(?<a>A)|(?<b>B), where the two capturing parentheses have
106 the same number but different names, is not supported, and causes an
107 error at compile time. If it were allowed, it would not be possible to
108 distinguish which parentheses matched, because both names map to cap‐
109 turing subpattern number 1. To avoid this confusing situation, an error
110 is given at compile time.
111 14. Perl used to recognize comments in some places that PCRE2 does not,
113 for example, between the ( and ? at the start of a subpattern. If the
114 /x modifier is set, Perl allowed white space between ( and ? though the
115 latest Perls give an error (for a while it was just deprecated). There
116 may still be some cases where Perl behaves differently.
117 15. Perl, when in warning mode, gives warnings for character classes
119 such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter‐
120 als. PCRE2 has no warning features, so it gives an error in these cases
121 because they are almost certainly user mistakes.
122 16. In PCRE2, the upper/lower case character properties Lu and Ll are
124 not affected when case-independent matching is specified. For example,
125 \p{Lu} always matches an upper case letter. I think Perl has changed in
126 this respect; in the release at the time of writing (5.16), \p{Lu} and
127 \p{Ll} match all letters, regardless of case, when case independence is
128 specified.
129 17. PCRE2 provides some extensions to the Perl regular expression
131 facilities. Perl 5.10 includes new features that are not in earlier
132 versions of Perl, some of which (such as named parentheses) have been
133 in PCRE2 for some time. This list is with respect to Perl 5.10:
134 (a) Although lookbehind assertions in PCRE2 must match fixed length
136 strings, each alternative branch of a lookbehind assertion can match a
137 different length of string. Perl requires them all to have the same
138 length.
139 (b) From PCRE2 10.23, back references to groups of fixed length are
141 supported in lookbehinds, provided that there is no possibility of ref‐
142 erencing a non-unique number or name. Perl does not support backrefer‐
143 ences in lookbehinds.
144 (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
146 $ meta-character matches only at the very end of the string.
147 (d) A backslash followed by a letter with no special meaning is
149 faulted. (Perl can be made to issue a warning.)
150 (e) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti‐
152 fiers is inverted, that is, by default they are not greedy, but if fol‐
153 lowed by a question mark they are.
154 (f) PCRE2_ANCHORED can be used at matching time to force a pattern to
156 be tried only at the first matching position in the subject string.
157 (g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
159 PCRE2_NOTEMPTY_ATSTART, and PCRE2_NO_AUTO_CAPTURE options have no Perl
160 equivalents.
161 (h) The \R escape sequence can be restricted to match only CR, LF, or
163 CRLF by the PCRE2_BSR_ANYCRLF option.
164 (i) The callout facility is PCRE2-specific.
166 (j) The partial matching facility is PCRE2-specific.
168 (k) The alternative matching function (pcre2_dfa_match() matches in a
170 different way and is not Perl-compatible.
171 (l) PCRE2 recognizes some special sequences such as (*CR) at the start
173 of a pattern that set overall options that cannot be changed within the
174 pattern.
175
177 Philip Hazel
179 University Computing Service
180 Cambridge, England.
181
183 Last updated: 18 October 2016
185 Copyright (c) 1997-2016 University of Cambridge.
186PCRE2 10.23 18 October 2016 PCRE2COMPAT(3)