1PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)
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NAME

6       PCRE2 - Perl-compatible regular expressions (revised API)
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DIFFERENCES BETWEEN PCRE2 AND PERL

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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13       ally changing, the information may sometimes be out of date.
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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.
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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.
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26       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).
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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 the PCRE2_ALT_BSUX
38       option is set, \U and \u are interpreted as ECMAScript interprets them.
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40       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."
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50       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:
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60           Pattern            PCRE2 matches     Perl matches
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62           \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
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69       The  \Q...\E  sequence  is recognized both inside and outside character
70       classes.
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72       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.
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77       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.
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81       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.
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90       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.
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96       11. Most backtracking verbs in assertions have  their  normal  actions.
97       They are not confined to the assertion.
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99       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".
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104       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.
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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 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.
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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.
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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.
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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:
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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.
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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.
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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.
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151       (d)  A  backslash  followed  by  a  letter  with  no special meaning is
152       faulted. (Perl can be made to issue a warning.)
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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.
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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.
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161       (g)     The     PCRE2_NOTBOL,    PCRE2_NOTEOL,    PCRE2_NOTEMPTY    and
162       PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
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164       (h) The \R escape sequence can be restricted to match only CR,  LF,  or
165       CRLF by the PCRE2_BSR_ANYCRLF option.
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167       (i)  The  callout  facility is PCRE2-specific. Perl supports codeblocks
168       and variable interpolation, but not general hooks on every match.
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170       (j) The partial matching facility is PCRE2-specific.
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172       (k) The alternative matching function (pcre2_dfa_match() matches  in  a
173       different way and is not Perl-compatible.
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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.
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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.
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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.
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AUTHOR

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

198
199       Last updated: 28 July 2018
200       Copyright (c) 1997-2018 University of Cambridge.
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204PCRE2 10.32                      28 July 2018                   PCRE2COMPAT(3)
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