Annotation of embedaddon/pcre/doc/pcreperform.3, revision 1.1.1.1
1.1 misho 1: .TH PCREPERFORM 3
2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "PCRE PERFORMANCE"
5: .rs
6: .sp
7: Two aspects of performance are discussed below: memory usage and processing
8: time. The way you express your pattern as a regular expression can affect both
9: of them.
10: .
11: .SH "COMPILED PATTERN MEMORY USAGE"
12: .rs
13: .sp
14: Patterns are compiled by PCRE into a reasonably efficient byte code, so that
15: most simple patterns do not use much memory. However, there is one case where
16: the memory usage of a compiled pattern can be unexpectedly large. If a
17: parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
18: a limited maximum, the whole subpattern is repeated in the compiled code. For
19: example, the pattern
20: .sp
21: (abc|def){2,4}
22: .sp
23: is compiled as if it were
24: .sp
25: (abc|def)(abc|def)((abc|def)(abc|def)?)?
26: .sp
27: (Technical aside: It is done this way so that backtrack points within each of
28: the repetitions can be independently maintained.)
29: .P
30: For regular expressions whose quantifiers use only small numbers, this is not
31: usually a problem. However, if the numbers are large, and particularly if such
32: repetitions are nested, the memory usage can become an embarrassment. For
33: example, the very simple pattern
34: .sp
35: ((ab){1,1000}c){1,3}
36: .sp
37: uses 51K bytes when compiled. When PCRE is compiled with its default internal
38: pointer size of two bytes, the size limit on a compiled pattern is 64K, and
39: this is reached with the above pattern if the outer repetition is increased
40: from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
41: handle larger compiled patterns, but it is better to try to rewrite your
42: pattern to use less memory if you can.
43: .P
44: One way of reducing the memory usage for such patterns is to make use of PCRE's
45: .\" HTML <a href="pcrepattern.html#subpatternsassubroutines">
46: .\" </a>
47: "subroutine"
48: .\"
49: facility. Re-writing the above pattern as
50: .sp
51: ((ab)(?2){0,999}c)(?1){0,2}
52: .sp
53: reduces the memory requirements to 18K, and indeed it remains under 20K even
54: with the outer repetition increased to 100. However, this pattern is not
55: exactly equivalent, because the "subroutine" calls are treated as
56: .\" HTML <a href="pcrepattern.html#atomicgroup">
57: .\" </a>
58: atomic groups
59: .\"
60: into which there can be no backtracking if there is a subsequent matching
61: failure. Therefore, PCRE cannot do this kind of rewriting automatically.
62: Furthermore, there is a noticeable loss of speed when executing the modified
63: pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
64: speed is acceptable, this kind of rewriting will allow you to process patterns
65: that PCRE cannot otherwise handle.
66: .
67: .
68: .SH "STACK USAGE AT RUN TIME"
69: .rs
70: .sp
71: When \fBpcre_exec()\fP is used for matching, certain kinds of pattern can cause
72: it to use large amounts of the process stack. In some environments the default
73: process stack is quite small, and if it runs out the result is often SIGSEGV.
74: This issue is probably the most frequently raised problem with PCRE. Rewriting
75: your pattern can often help. The
76: .\" HREF
77: \fBpcrestack\fP
78: .\"
79: documentation discusses this issue in detail.
80: .
81: .
82: .SH "PROCESSING TIME"
83: .rs
84: .sp
85: Certain items in regular expression patterns are processed more efficiently
86: than others. It is more efficient to use a character class like [aeiou] than a
87: set of single-character alternatives such as (a|e|i|o|u). In general, the
88: simplest construction that provides the required behaviour is usually the most
89: efficient. Jeffrey Friedl's book contains a lot of useful general discussion
90: about optimizing regular expressions for efficient performance. This document
91: contains a few observations about PCRE.
92: .P
93: Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
94: because PCRE has to scan a structure that contains data for over fifteen
95: thousand characters whenever it needs a character's property. If you can find
96: an alternative pattern that does not use character properties, it will probably
97: be faster.
98: .P
99: By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX
100: character classes such as [:alpha:] do not use Unicode properties, partly for
101: backwards compatibility, and partly for performance reasons. However, you can
102: set PCRE_UCP if you want Unicode character properties to be used. This can
103: double the matching time for items such as \ed, when matched with
104: \fBpcre_exec()\fP; the performance loss is less with \fBpcre_dfa_exec()\fP, and
105: in both cases there is not much difference for \eb.
106: .P
107: When a pattern begins with .* not in parentheses, or in parentheses that are
108: not the subject of a backreference, and the PCRE_DOTALL option is set, the
109: pattern is implicitly anchored by PCRE, since it can match only at the start of
110: a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
111: optimization, because the . metacharacter does not then match a newline, and if
112: the subject string contains newlines, the pattern may match from the character
113: immediately following one of them instead of from the very start. For example,
114: the pattern
115: .sp
116: .*second
117: .sp
118: matches the subject "first\enand second" (where \en stands for a newline
119: character), with the match starting at the seventh character. In order to do
120: this, PCRE has to retry the match starting after every newline in the subject.
121: .P
122: If you are using such a pattern with subject strings that do not contain
123: newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
124: the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
125: from having to scan along the subject looking for a newline to restart at.
126: .P
127: Beware of patterns that contain nested indefinite repeats. These can take a
128: long time to run when applied to a string that does not match. Consider the
129: pattern fragment
130: .sp
131: ^(a+)*
132: .sp
133: This can match "aaaa" in 16 different ways, and this number increases very
134: rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
135: times, and for each of those cases other than 0 or 4, the + repeats can match
136: different numbers of times.) When the remainder of the pattern is such that the
137: entire match is going to fail, PCRE has in principle to try every possible
138: variation, and this can take an extremely long time, even for relatively short
139: strings.
140: .P
141: An optimization catches some of the more simple cases such as
142: .sp
143: (a+)*b
144: .sp
145: where a literal character follows. Before embarking on the standard matching
146: procedure, PCRE checks that there is a "b" later in the subject string, and if
147: there is not, it fails the match immediately. However, when there is no
148: following literal this optimization cannot be used. You can see the difference
149: by comparing the behaviour of
150: .sp
151: (a+)*\ed
152: .sp
153: with the pattern above. The former gives a failure almost instantly when
154: applied to a whole line of "a" characters, whereas the latter takes an
155: appreciable time with strings longer than about 20 characters.
156: .P
157: In many cases, the solution to this kind of performance issue is to use an
158: atomic group or a possessive quantifier.
159: .
160: .
161: .SH AUTHOR
162: .rs
163: .sp
164: .nf
165: Philip Hazel
166: University Computing Service
167: Cambridge CB2 3QH, England.
168: .fi
169: .
170: .
171: .SH REVISION
172: .rs
173: .sp
174: .nf
175: Last updated: 16 May 2010
176: Copyright (c) 1997-2010 University of Cambridge.
177: .fi
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