1PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)
2
3
4

NAME

6       PCRE2 - Perl-compatible regular expressions (revised API)
7

DIFFERENCES BETWEEN PCRE2 AND PERL

9
10       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13       ally changing, the information may sometimes be out of date.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
15       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
18       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
26       3.  Capture groups that occur inside negative lookaround assertions are
27       counted, but their entries in the offsets vector are set  only  when  a
28       negative  assertion is a condition that has a matching branch (that is,
29       the condition is false).
30
31       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  either  of  the
38       PCRE2_ALT_BSUX  or  PCRE2_EXTRA_ALT_BSUX  options is set, \U and \u are
39       interpreted as ECMAScript interprets them.
40
41       5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
42       is built with Unicode support (the default). The properties that can be
43       tested with \p and \P are limited to the  general  category  properties
44       such  as  Lu and Nd, script names such as Greek or Han, and the derived
45       properties Any and L&.  PCRE2 does support the Cs (surrogate) property,
46       which  Perl  does  not; the Perl documentation says "Because Perl hides
47       the need for the user to understand the internal representation of Uni‐
48       code  characters, there is no need to implement the somewhat messy con‐
49       cept of surrogates."
50
51       6. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
52       in between are treated as literals. However, this is slightly different
53       from Perl in that $ and @ are  also  handled  as  literals  inside  the
54       quotes. In Perl, they cause variable interpolation (but of course PCRE2
55       does not have variables). Also,  Perl  does  "double-quotish  backslash
56       interpolation" on any backslashes between \Q and \E which, its documen‐
57       tation says, "may lead to confusing results". PCRE2 treats a  backslash
58       between  \Q  and  \E  just like any other character. Note the following
59       examples:
60
61           Pattern            PCRE2 matches     Perl matches
62
63           \Qabc$xyz\E        abc$xyz           abc followed by the
64                                                  contents of $xyz
65           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
66           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
67           \QA\B\E            A\B               A\B
68           \Q\\E              \                 \\E
69
70       The \Q...\E sequence is recognized both inside  and  outside  character
71       classes.
72
73       7.   Fairly  obviously,  PCRE2  does  not  support  the  (?{code})  and
74       (??{code}) constructions. However, PCRE2 does have a "callout" feature,
75       which allows an external function to be called during pattern matching.
76       See the pcre2callout documentation for details.
77
78       8. Subroutine calls (whether recursive or not) were treated  as  atomic
79       groups  up to PCRE2 release 10.23, but from release 10.30 this changed,
80       and backtracking into subroutine calls is now supported, as in Perl.
81
82       9. If any of the backtracking control verbs are used in a group that is
83       called  as  a  subroutine (whether or not recursively), their effect is
84       confined to that group; it does not extend to the surrounding  pattern.
85       This  is  not  always  the  case  in Perl. In particular, if (*THEN) is
86       present in a group that is called as a subroutine, its action  is  lim‐
87       ited  to  that  group, even if the group does not contain any | charac‐
88       ters. Note that such groups are processed  as  anchored  at  the  point
89       where they are tested.
90
91       10.  If a pattern contains more than one backtracking control verb, the
92       first one that is backtracked onto acts. For example,  in  the  pattern
93       A(*COMMIT)B(*PRUNE)C  a  failure in B triggers (*COMMIT), but a failure
94       in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
95       it is the same as PCRE2, but there are cases where it differs.
96
97       11.  Most  backtracking  verbs in assertions have their normal actions.
98       They are not confined to the assertion.
99
100       12. 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
105       13.  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
115       14. 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
121       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
126       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
133       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
138       (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
143       (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
148       (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
151       (d)  A  backslash  followed  by  a  letter  with  no special meaning is
152       faulted. (Perl can be made to issue a warning.)
153
154       (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
158       (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
161       (g)     The     PCRE2_NOTBOL,    PCRE2_NOTEOL,    PCRE2_NOTEMPTY    and
162       PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
163
164       (h) The \R escape sequence can be restricted to match only CR,  LF,  or
165       CRLF by the PCRE2_BSR_ANYCRLF option.
166
167       (i)  The  callout  facility is PCRE2-specific. Perl supports codeblocks
168       and variable interpolation, but not general hooks on every match.
169
170       (j) The partial matching facility is PCRE2-specific.
171
172       (k) The alternative matching function (pcre2_dfa_match() matches  in  a
173       different way and is not Perl-compatible.
174
175       (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
179       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
184       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

AUTHOR

192
193       Philip Hazel
194       University Computing Service
195       Cambridge, England.
196

REVISION

198
199       Last updated: 12 February 2019
200       Copyright (c) 1997-2019 University of Cambridge.
201
202
203
204PCRE2 10.33                    12 February 2019                 PCRE2COMPAT(3)
Impressum