Annotation of embedaddon/pcre/doc/pcrecompat.3, revision 1.1.1.4
1.1.1.4 ! misho 1: .TH PCRECOMPAT 3 "19 March 2013" "PCRE 8.33"
1.1 misho 2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "DIFFERENCES BETWEEN PCRE AND PERL"
5: .rs
6: .sp
7: 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
9: versions 5.10 and above.
10: .P
1.1.1.2 misho 11: 1. PCRE has only a subset of Perl's Unicode support. Details of what it does
12: have are given in the
1.1 misho 13: .\" HREF
14: \fBpcreunicode\fP
15: .\"
16: page.
17: .P
18: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
19: not mean what you might think. For example, (?!a){3} does not assert that the
20: next three characters are not "a". It just asserts that the next character is
21: not "a" three times (in principle: PCRE optimizes this to run the assertion
22: just once). Perl allows repeat quantifiers on other assertions such as \eb, but
23: these do not seem to have any use.
24: .P
25: 3. Capturing subpatterns that occur inside negative lookahead assertions are
1.1.1.4 ! misho 26: counted, but their entries in the offsets vector are never set. Perl sometimes
! 27: (but not always) sets its numerical variables from inside negative assertions.
1.1 misho 28: .P
29: 4. Though binary zero characters are supported in the subject string, they are
30: not allowed in a pattern string because it is passed as a normal C string,
31: terminated by zero. The escape sequence \e0 can be used in the pattern to
32: represent a binary zero.
33: .P
34: 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
35: \eU, and \eN when followed by a character name or Unicode value. (\eN on its
36: own, matching a non-newline character, is supported.) In fact these are
37: implemented by Perl's general string-handling and are not part of its pattern
38: matching engine. If any of these are encountered by PCRE, an error is
39: generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
40: \eU and \eu are interpreted as JavaScript interprets them.
41: .P
42: 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
43: built with Unicode character property support. The properties that can be
44: tested with \ep and \eP are limited to the general category properties such as
45: Lu and Nd, script names such as Greek or Han, and the derived properties Any
46: and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
47: Perl documentation says "Because Perl hides the need for the user to understand
48: the internal representation of Unicode characters, there is no need to
49: implement the somewhat messy concept of surrogates."
50: .P
1.1.1.4 ! misho 51: 7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
1.1 misho 52: between are treated as literals. This is slightly different from Perl in that $
53: and @ are also handled as literals inside the quotes. In Perl, they cause
54: variable interpolation (but of course PCRE does not have variables). Note the
55: following examples:
56: .sp
57: Pattern PCRE matches Perl matches
58: .sp
59: .\" JOIN
60: \eQabc$xyz\eE abc$xyz abc followed by the
61: contents of $xyz
62: \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
63: \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
64: .sp
65: The \eQ...\eE sequence is recognized both inside and outside character classes.
66: .P
1.1.1.4 ! misho 67: 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
1.1 misho 68: constructions. However, there is support for recursive patterns. This is not
69: available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
70: feature allows an external function to be called during pattern matching. See
71: the
72: .\" HREF
73: \fBpcrecallout\fP
74: .\"
75: documentation for details.
76: .P
1.1.1.4 ! misho 77: 9. Subpatterns that are called as subroutines (whether or not recursively) are
1.1 misho 78: always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
79: Captured values that are set outside a subroutine call can be reference from
80: inside in PCRE, but not in Perl. There is a discussion that explains these
81: differences 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
1.1.1.4 ! misho 92: 10. If any of the backtracking control verbs are used in a subpattern that is
! 93: called as a subroutine (whether or not recursively), their effect is confined
! 94: to that subpattern; it does not extend to the surrounding pattern. This is not
! 95: always the case in Perl. In particular, if (*THEN) is present in a group that
! 96: is called as a subroutine, its action is limited to that group, even if the
! 97: group does not contain any | characters. Note that such subpatterns are
! 98: processed as anchored at the point where they are tested.
! 99: .P
! 100: 11. If a pattern contains more than one backtracking control verb, the first
! 101: one that is backtracked onto acts. For example, in the pattern
! 102: A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
! 103: triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
! 104: same as PCRE, but there are examples where it differs.
! 105: .P
! 106: 12. Most backtracking verbs in assertions have their normal actions. They are
! 107: not confined to the assertion.
1.1 misho 108: .P
1.1.1.4 ! misho 109: 13. There are some differences that are concerned with the settings of captured
1.1 misho 110: strings when part of a pattern is repeated. For example, matching "aba" against
111: the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
112: .P
1.1.1.4 ! misho 113: 14. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
1.1 misho 114: names is not as general as Perl's. This is a consequence of the fact the PCRE
115: works internally just with numbers, using an external table to translate
116: between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
117: where the two capturing parentheses have the same number but different names,
118: is not supported, and causes an error at compile time. If it were allowed, it
119: would not be possible to distinguish which parentheses matched, because both
120: names map to capturing subpattern number 1. To avoid this confusing situation,
121: an error is given at compile time.
122: .P
1.1.1.4 ! misho 123: 15. Perl recognizes comments in some places that PCRE does not, for example,
1.1 misho 124: between the ( and ? at the start of a subpattern. If the /x modifier is set,
1.1.1.3 misho 125: Perl allows white space between ( and ? but PCRE never does, even if the
1.1 misho 126: PCRE_EXTENDED option is set.
127: .P
1.1.1.4 ! misho 128: 16. In PCRE, the upper/lower case character properties Lu and Ll are not
! 129: affected when case-independent matching is specified. For example, \ep{Lu}
! 130: always matches an upper case letter. I think Perl has changed in this respect;
! 131: in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all
! 132: letters, regardless of case, when case independence is specified.
! 133: .P
! 134: 17. PCRE provides some extensions to the Perl regular expression facilities.
1.1 misho 135: Perl 5.10 includes new features that are not in earlier versions of Perl, some
136: of which (such as named parentheses) have been in PCRE for some time. This list
137: is with respect to Perl 5.10:
138: .sp
139: (a) Although lookbehind assertions in PCRE must match fixed length strings,
140: each alternative branch of a lookbehind assertion can match a different length
141: of string. Perl requires them all to have the same length.
142: .sp
143: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
144: meta-character matches only at the very end of the string.
145: .sp
146: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
147: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
148: (Perl can be made to issue a warning.)
149: .sp
150: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
151: inverted, that is, by default they are not greedy, but if followed by a
152: question mark they are.
153: .sp
154: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
155: only at the first matching position in the subject string.
156: .sp
157: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
158: PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
159: .sp
160: (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
161: by the PCRE_BSR_ANYCRLF option.
162: .sp
163: (h) The callout facility is PCRE-specific.
164: .sp
165: (i) The partial matching facility is PCRE-specific.
166: .sp
167: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
168: different hosts that have the other endianness. However, this does not apply to
169: optimized data created by the just-in-time compiler.
170: .sp
1.1.1.4 ! misho 171: (k) The alternative matching functions (\fBpcre_dfa_exec()\fP,
! 172: \fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP,) match in a different way
! 173: and are not Perl-compatible.
1.1 misho 174: .sp
175: (l) PCRE recognizes some special sequences such as (*CR) at the start of
176: a pattern that set overall options that cannot be changed within the pattern.
177: .
178: .
179: .SH AUTHOR
180: .rs
181: .sp
182: .nf
183: Philip Hazel
184: University Computing Service
185: Cambridge CB2 3QH, England.
186: .fi
187: .
188: .
189: .SH REVISION
190: .rs
191: .sp
192: .nf
1.1.1.4 ! misho 193: Last updated: 19 March 2013
! 194: Copyright (c) 1997-2013 University of Cambridge.
1.1 misho 195: .fi
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