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revision 63 by nigel, Sat Feb 24 21:40:03 2007 UTC revision 635 by ph10, Sat Jul 23 16:19:50 2011 UTC
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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.  versions 5.10 and above.
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  them, but they do not mean what you might think. For example, (?!a){3} does  it does have are given in the
13  not assert that the next three characters are not "a". It just asserts that the  .\" HTML <a href="pcre.html#utf8support">
14  next character is not "a" three times.  .\" </a>
15    section on UTF-8 support
16  2. Capturing subpatterns that occur inside negative lookahead assertions are  .\"
17    in the main
18    .\" HREF
19    \fBpcre\fP
20    .\"
21    page.
22    .P
23    2. PCRE does not allow repeat quantifiers on assertions. Perl permits them, but
24    they do not mean what you might think. For example, (?!a){3} does not assert
25    that the next three characters are not "a". It just asserts that the next
26    character is not "a" three times.
27    .P
28    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 when followed by a character name or Unicode value. (\eN on its
41  string-handling and are not part of its pattern matching engine. If any of  own, matching a non-newline character, is supported.) In fact these are
42  these are encountered by PCRE, an error is generated.  implemented by Perl's general string-handling and are not part of its pattern
43    matching engine. If any of these are encountered by PCRE, an error is
44  5. PCRE does support the \\Q...\\E escape for quoting substrings. Characters in  generated.
45    .P
46    6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
47    built with Unicode character property support. The properties that can be
48    tested with \ep and \eP are limited to the general category properties such as
49    Lu and Nd, script names such as Greek or Han, and the derived properties Any
50    and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
51    Perl documentation says "Because Perl hides the need for the user to understand
52    the internal representation of Unicode characters, there is no need to
53    implement the somewhat messy concept of surrogates."
54    .P
55    7. PCRE implements a simpler version of \eX than Perl, which changed to make
56    \eX match what Unicode calls an "extended grapheme cluster". This is more
57    complicated than an extended Unicode sequence, which is what PCRE matches.
58    .P
59    8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
60  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 $
61  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
62  variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
63  following examples:  following examples:
64    .sp
65      Pattern            PCRE matches      Perl matches      Pattern            PCRE matches      Perl matches
66    .sp
67      \\Qabc$xyz\\E        abc$xyz           abc followed by the  .\" JOIN
68        \eQabc$xyz\eE        abc$xyz           abc followed by the
69                                             contents of $xyz                                             contents of $xyz
70      \\Qabc\\$xyz\\E       abc\\$xyz          abc\\$xyz      \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
71      \\Qabc\\E\\$\\Qxyz\\E   abc$xyz           abc$xyz      \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
72    .sp
73  In PCRE, the \\Q...\\E mechanism is not recognized inside a character class.  The \eQ...\eE sequence is recognized both inside and outside character classes.
74    .P
75  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
76  constructions. However, there is some experimental support for recursive  constructions. However, there is support for recursive patterns. This is not
77  patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the PCRE  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
78  "callout" feature allows an external function to be called during pattern  feature allows an external function to be called during pattern matching. See
79  matching.  the
80    .\" HREF
81  9. There are some differences that are concerned with the settings of captured  \fBpcrecallout\fP
82    .\"
83    documentation for details.
84    .P
85    10. Subpatterns that are called recursively or as "subroutines" are always
86    treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
87    is a discussion of an example that explains this in more detail in the
88    .\" HTML <a href="pcrepattern.html#recursiondifference">
89    .\" </a>
90    section on recursion differences from Perl
91    .\"
92    in the
93    .\" HREF
94    \fBpcrepattern\fP
95    .\"
96    page.
97    .P
98    11. There are some differences that are concerned with the settings of captured
99  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
100  the pattern /^(a(b)?)+$/ 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".
101    .P
102  10. PCRE provides some extensions to the Perl regular expression facilities:  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
103    names is not as general as Perl's. This is a consequence of the fact the PCRE
104  (a) Although lookbehind assertions must match fixed length strings, each  works internally just with numbers, using an external table to translate
105  alternative branch of a lookbehind assertion can match a different length of  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
106  string. Perl requires them all to have the same length.  where the two capturing parentheses have the same number but different names,
107    is not supported, and causes an error at compile time. If it were allowed, it
108    would not be possible to distinguish which parentheses matched, because both
109    names map to capturing subpattern number 1. To avoid this confusing situation,
110    an error is given at compile time.
111    .P
112    13. Perl recognizes comments in some places that PCRE does not, for example,
113    between the ( and ? at the start of a subpattern. If the /x modifier is set,
114    Perl allows whitespace between ( and ? but PCRE never does, even if the
115    PCRE_EXTENDED option is set.
116    .P
117    14. PCRE provides some extensions to the Perl regular expression facilities.
118    Perl 5.10 includes new features that are not in earlier versions of Perl, some
119    of which (such as named parentheses) have been in PCRE for some time. This list
120    is with respect to Perl 5.10:
121    .sp
122    (a) Although lookbehind assertions in PCRE must match fixed length strings,
123    each alternative branch of a lookbehind assertion can match a different length
124    of string. Perl requires them all to have the same length.
125    .sp
126  (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 $
127  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
128    .sp
129  (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
130  meaning is faulted.  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
131    (Perl can be made to issue a warning.)
132    .sp
133  (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
134  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
135  question mark they are.  question mark they are.
136    .sp
137  (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
138  matching position in the subject string.  only at the first matching position in the subject string.
139    .sp
140  (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
141  options for \fBpcre_exec()\fR have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
142    .sp
143  (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
144  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
145  support.)  .sp
146    (h) The callout facility is PCRE-specific.
147  (h) PCRE supports named capturing substrings, using the Python syntax.  .sp
148    (i) The partial matching facility is PCRE-specific.
149  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  .sp
150  package.  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
151    different hosts that have the other endianness.
152  (j) The (R) condition, for testing recursion, is a PCRE extension.  .sp
153    (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
154  (k) The callout facility is PCRE-specific.  different way and is not Perl-compatible.
155    .sp
156  .in 0  (l) PCRE recognizes some special sequences such as (*CR) at the start of
157  Last updated: 03 February 2003  a pattern that set overall options that cannot be changed within the pattern.
158  .br  .
159  Copyright (c) 1997-2003 University of Cambridge.  .
160    .SH AUTHOR
161    .rs
162    .sp
163    .nf
164    Philip Hazel
165    University Computing Service
166    Cambridge CB2 3QH, England.
167    .fi
168    .
169    .
170    .SH REVISION
171    .rs
172    .sp
173    .nf
174    Last updated: 23 July 2011
175    Copyright (c) 1997-2011 University of Cambridge.
176    .fi

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