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1  .TH PCRE 3  .TH PCRECOMPAT 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH DIFFERENCES FROM PERL  .SH "DIFFERENCES BETWEEN PCRE AND PERL"
5  .rs  .rs
6  .sp  .sp
7  This document describes the differences in the ways that PCRE and Perl handle  This document describes the differences in the ways that PCRE and Perl handle
8  regular expressions. The differences described here are with respect to Perl  regular expressions. The differences described here are with respect to Perl
9  5.8.  5.10.
10    .P
11  1. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12    it does have are given in the
13    .\" HTML <a href="pcre.html#utf8support">
14    .\" </a>
15    section on UTF-8 support
16    .\"
17    in the main
18    .\" HREF
19    \fBpcre\fP
20    .\"
21    page.
22    .P
23    2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
24  them, but they do not mean what you might think. For example, (?!a){3} does  them, but they do not mean what you might think. For example, (?!a){3} does
25  not assert that the next three characters are not "a". It just asserts that the  not assert that the next three characters are not "a". It just asserts that the
26  next character is not "a" three times.  next character is not "a" three times.
27    .P
28  2. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
29  counted, but their entries in the offsets vector are never set. Perl sets its  counted, but their entries in the offsets vector are never set. Perl sets its
30  numerical variables from any such patterns that are matched before the  numerical variables from any such patterns that are matched before the
31  assertion fails to match something (thereby succeeding), but only if the  assertion fails to match something (thereby succeeding), but only if the
32  negative lookahead assertion contains just one branch.  negative lookahead assertion contains just one branch.
33    .P
34  3. Though binary zero characters are supported in the subject string, they are  4. Though binary zero characters are supported in the subject string, they are
35  not allowed in a pattern string because it is passed as a normal C string,  not allowed in a pattern string because it is passed as a normal C string,
36  terminated by zero. The escape sequence "\\0" can be used in the pattern to  terminated by zero. The escape sequence \e0 can be used in the pattern to
37  represent a binary zero.  represent a binary zero.
38    .P
39  4. The following Perl escape sequences are not supported: \\l, \\u, \\L,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
40  \\U, \\P, \\p, \N, and \\X. In fact these are implemented by Perl's general  \eU, and \eN. In fact these are implemented by Perl's general string-handling
41  string-handling and are not part of its pattern matching engine. If any of  and are not part of its pattern matching engine. If any of these are
42  these are encountered by PCRE, an error is generated.  encountered by PCRE, an error is generated.
43    .P
44  5. PCRE does support the \\Q...\\E escape for quoting substrings. Characters in  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
45    built with Unicode character property support. The properties that can be
46    tested with \ep and \eP are limited to the general category properties such as
47    Lu and Nd, script names such as Greek or Han, and the derived properties Any
48    and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
49    Perl documentation says "Because Perl hides the need for the user to understand
50    the internal representation of Unicode characters, there is no need to
51    implement the somewhat messy concept of surrogates."
52    .P
53    7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
54  between are treated as literals. This is slightly different from Perl in that $  between are treated as literals. This is slightly different from Perl in that $
55  and @ are also handled as literals inside the quotes. In Perl, they cause  and @ are also handled as literals inside the quotes. In Perl, they cause
56  variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
57  following examples:  following examples:
58    .sp
59      Pattern            PCRE matches      Perl matches      Pattern            PCRE matches      Perl matches
60    .sp
61      \\Qabc$xyz\\E        abc$xyz           abc followed by the  .\" JOIN
62        \eQabc$xyz\eE        abc$xyz           abc followed by the
63                                             contents of $xyz                                             contents of $xyz
64      \\Qabc\\$xyz\\E       abc\\$xyz          abc\\$xyz      \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
65      \\Qabc\\E\\$\\Qxyz\\E   abc$xyz           abc$xyz      \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
66    .sp
67  In PCRE, the \\Q...\\E mechanism is not recognized inside a character class.  The \eQ...\eE sequence is recognized both inside and outside character classes.
68    .P
69  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70  constructions. However, there is some experimental support for recursive  constructions. However, there is support for recursive patterns. This is not
71  patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the PCRE  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72  "callout" feature allows an external function to be called during pattern  feature allows an external function to be called during pattern matching. See
73  matching.  the
74    .\" HREF
75  9. There are some differences that are concerned with the settings of captured  \fBpcrecallout\fP
76    .\"
77    documentation for details.
78    .P
79    9. Subpatterns that are called recursively or as "subroutines" are always
80    treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81    is a discussion of an example that explains this in more detail in the
82    .\" HTML <a href="pcrepattern.html#recursiondifference">
83    .\" </a>
84    section on recursion differences from Perl
85    .\"
86    in the
87    .\" HREF
88    \fBpcrepattern\fP
89    .\"
90    page.
91    .P
92    10. There are some differences that are concerned with the settings of captured
93  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
94  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
95    .P
96  10. PCRE provides some extensions to the Perl regular expression facilities:  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
97    (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
98  (a) Although lookbehind assertions must match fixed length strings, each  argument. PCRE does not support (*MARK).
99  alternative branch of a lookbehind assertion can match a different length of  .P
100  string. Perl requires them all to have the same length.  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101    names is not as general as Perl's. This is a consequence of the fact the PCRE
102    works internally just with numbers, using an external table to translate
103    between numbers and names. The following are some specific differences:
104    .sp
105    (a) After matching a pattern such as (?|(?<a>A)|(?<b)B) where the two capturing
106    parentheses have the same number but different names, it is not possible to
107    distinguish which parentheses matched, because both names map to capturing
108    subpattern number 1.
109    .sp
110    (b) A condition test for a subpattern with a name that is duplicated gives
111    unpredictable results. For example, when the pattern
112    (?:(?<a>A)|(?<a>B))(?('a')...|...) is compiled (the PCRE_DUPNAMES option is
113    required), the condition test (?('a') is set to test whether subpattern 1 has
114    matched, ignoring subpattern 2, even though it has the same name.
115    .P
116    13. PCRE provides some extensions to the Perl regular expression facilities.
117    Perl 5.10 includes new features that are not in earlier versions of Perl, some
118    of which (such as named parentheses) have been in PCRE for some time. This list
119    is with respect to Perl 5.10:
120    .sp
121    (a) Although lookbehind assertions in PCRE must match fixed length strings,
122    each alternative branch of a lookbehind assertion can match a different length
123    of string. Perl requires them all to have the same length.
124    .sp
125  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
126  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
127    .sp
128  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
129  meaning is faulted.  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
130    (Perl can be made to issue a warning.)
131    .sp
132  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
133  inverted, that is, by default they are not greedy, but if followed by a  inverted, that is, by default they are not greedy, but if followed by a
134  question mark they are.  question mark they are.
135    .sp
136  (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
137  matching position in the subject string.  only at the first matching position in the subject string.
138    .sp
139  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
140  options for \fBpcre_exec()\fR have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
141    .sp
142  (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
143  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
144  support.)  .sp
145    (h) The callout facility is PCRE-specific.
146  (h) PCRE supports named capturing substrings, using the Python syntax.  .sp
147    (i) The partial matching facility is PCRE-specific.
148  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  .sp
149  package.  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
150    different hosts that have the other endianness.
151  (j) The (R) condition, for testing recursion, is a PCRE extension.  .sp
152    (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
153  (k) The callout facility is PCRE-specific.  different way and is not Perl-compatible.
154    .sp
155  .in 0  (l) PCRE recognizes some special sequences such as (*CR) at the start of
156  Last updated: 03 February 2003  a pattern that set overall options that cannot be changed within the pattern.
157  .br  .
158  Copyright (c) 1997-2003 University of Cambridge.  .
159    .SH AUTHOR
160    .rs
161    .sp
162    .nf
163    Philip Hazel
164    University Computing Service
165    Cambridge CB2 3QH, England.
166    .fi
167    .
168    .
169    .SH REVISION
170    .rs
171    .sp
172    .nf
173    Last updated: 29 September 2009
174    Copyright (c) 1997-2009 University of Cambridge.
175    .fi

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