trunk/doc/pcreperform.3

.TH PCREPERFORM 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE PERFORMANCE"
.rs
.sp
Two aspects of performance are discussed below: memory usage and processing
time. The way you express your pattern as a regular expression can affect both
of them.
.
.SH "COMPILED PATTERN MEMORY USAGE"
.rs
.sp
Patterns are compiled by PCRE into a reasonably efficient interpretive code, so
that most simple patterns do not use much memory. However, there is one case
where the memory usage of a compiled pattern can be unexpectedly large. If a
parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
a limited maximum, the whole subpattern is repeated in the compiled code. For
example, the pattern
.sp
  (abc|def){2,4}
.sp
is compiled as if it were
.sp
  (abc|def)(abc|def)((abc|def)(abc|def)?)?
.sp
(Technical aside: It is done this way so that backtrack points within each of
the repetitions can be independently maintained.)
.P
For regular expressions whose quantifiers use only small numbers, this is not
usually a problem. However, if the numbers are large, and particularly if such
repetitions are nested, the memory usage can become an embarrassment. For
example, the very simple pattern
.sp
  ((ab){1,1000}c){1,3}
.sp
uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
with its default internal pointer size of two bytes, the size limit on a
compiled pattern is 64K data units, and this is reached with the above pattern
if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
larger internal pointers and thus handle larger compiled patterns, but it is
better to try to rewrite your pattern to use less memory if you can.
.P
One way of reducing the memory usage for such patterns is to make use of PCRE's
.\" HTML <a href="pcrepattern.html#subpatternsassubroutines">
.\" </a>
"subroutine"
.\"
facility. Re-writing the above pattern as
.sp
  ((ab)(?2){0,999}c)(?1){0,2}
.sp
reduces the memory requirements to 18K, and indeed it remains under 20K even
with the outer repetition increased to 100. However, this pattern is not
exactly equivalent, because the "subroutine" calls are treated as
.\" HTML <a href="pcrepattern.html#atomicgroup">
.\" </a>
atomic groups
.\"
into which there can be no backtracking if there is a subsequent matching
failure. Therefore, PCRE cannot do this kind of rewriting automatically.
Furthermore, there is a noticeable loss of speed when executing the modified
pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
speed is acceptable, this kind of rewriting will allow you to process patterns
that PCRE cannot otherwise handle.
.
.
.SH "STACK USAGE AT RUN TIME"
.rs
.sp
When \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used for matching, certain
kinds of pattern can cause it to use large amounts of the process stack. In
some environments the default process stack is quite small, and if it runs out
the result is often SIGSEGV. This issue is probably the most frequently raised
problem with PCRE. Rewriting your pattern can often help. The
.\" HREF
\fBpcrestack\fP
.\"
documentation discusses this issue in detail.
.
.
.SH "PROCESSING TIME"
.rs
.sp
Certain items in regular expression patterns are processed more efficiently
than others. It is more efficient to use a character class like [aeiou] than a
set of single-character alternatives such as (a|e|i|o|u). In general, the
simplest construction that provides the required behaviour is usually the most
efficient. Jeffrey Friedl's book contains a lot of useful general discussion
about optimizing regular expressions for efficient performance. This document
contains a few observations about PCRE.
.P
Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
because PCRE has to scan a structure that contains data for over fifteen
thousand characters whenever it needs a character's property. If you can find
an alternative pattern that does not use character properties, it will probably
be faster.
.P
By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX
character classes such as [:alpha:] do not use Unicode properties, partly for
backwards compatibility, and partly for performance reasons. However, you can
set PCRE_UCP if you want Unicode character properties to be used. This can
double the matching time for items such as \ed, when matched with
a traditional matching function; the performance loss is less with
a DFA matching function, and in both cases there is not much difference for
\eb.
.P
When a pattern begins with .* not in parentheses, or in parentheses that are
not the subject of a backreference, and the PCRE_DOTALL option is set, the
pattern is implicitly anchored by PCRE, since it can match only at the start of
a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
optimization, because the . metacharacter does not then match a newline, and if
the subject string contains newlines, the pattern may match from the character
immediately following one of them instead of from the very start. For example,
the pattern
.sp
  .*second
.sp
matches the subject "first\enand second" (where \en stands for a newline
character), with the match starting at the seventh character. In order to do
this, PCRE has to retry the match starting after every newline in the subject.
.P
If you are using such a pattern with subject strings that do not contain
newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
from having to scan along the subject looking for a newline to restart at.
.P
Beware of patterns that contain nested indefinite repeats. These can take a
long time to run when applied to a string that does not match. Consider the
pattern fragment
.sp
  ^(a+)*
.sp
This can match "aaaa" in 16 different ways, and this number increases very
rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
times, and for each of those cases other than 0 or 4, the + repeats can match
different numbers of times.) When the remainder of the pattern is such that the
entire match is going to fail, PCRE has in principle to try every possible
variation, and this can take an extremely long time, even for relatively short
strings.
.P
An optimization catches some of the more simple cases such as
.sp
  (a+)*b
.sp
where a literal character follows. Before embarking on the standard matching
procedure, PCRE checks that there is a "b" later in the subject string, and if
there is not, it fails the match immediately. However, when there is no
following literal this optimization cannot be used. You can see the difference
by comparing the behaviour of
.sp
  (a+)*\ed
.sp
with the pattern above. The former gives a failure almost instantly when
applied to a whole line of "a" characters, whereas the latter takes an
appreciable time with strings longer than about 20 characters.
.P
In many cases, the solution to this kind of performance issue is to use an
atomic group or a possessive quantifier.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 09 January 2012
Copyright (c) 1997-2012 University of Cambridge.
.fi
1	nigel	79	.TH PCREPERFORM 3
2	nigel	63	.SH NAME
3			PCRE - Perl-compatible regular expressions
4	nigel	75	.SH "PCRE PERFORMANCE"
5	nigel	63	.rs
6			.sp
7	nigel	93	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	ph10	502	.SH "COMPILED PATTERN MEMORY USAGE"
12	nigel	93	.rs
13			.sp
14	ph10	859	Patterns are compiled by PCRE into a reasonably efficient interpretive code, so
15			that most simple patterns do not use much memory. However, there is one case
16			where the memory usage of a compiled pattern can be unexpectedly large. If a
17	ph10	502	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	nigel	93	.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	ph10	859	uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
38			with its default internal pointer size of two bytes, the size limit on a
39			compiled pattern is 64K data units, and this is reached with the above pattern
40			if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
41			larger internal pointers and thus handle larger compiled patterns, but it is
42			better to try to rewrite your pattern to use less memory if you can.
43	nigel	93	.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	ph10	502	.
68			.SH "STACK USAGE AT RUN TIME"
69			.rs
70			.sp
71	ph10	859	When \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used for matching, certain
72			kinds of pattern can cause it to use large amounts of the process stack. In
73			some environments the default process stack is quite small, and if it runs out
74			the result is often SIGSEGV. This issue is probably the most frequently raised
75			problem with PCRE. Rewriting your pattern can often help. The
76	ph10	502	.\" HREF
77			\fBpcrestack\fP
78			.\"
79			documentation discusses this issue in detail.
80			.
81			.
82	nigel	93	.SH "PROCESSING TIME"
83			.rs
84			.sp
85			Certain items in regular expression patterns are processed more efficiently
86	nigel	63	than others. It is more efficient to use a character class like [aeiou] than a
87	nigel	93	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	nigel	75	.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	ph10	535	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	ph10	859	a traditional matching function; the performance loss is less with
105			a DFA matching function, and in both cases there is not much difference for
106			\eb.
107	ph10	518	.P
108	nigel	63	When a pattern begins with .* not in parentheses, or in parentheses that are
109			not the subject of a backreference, and the PCRE_DOTALL option is set, the
110			pattern is implicitly anchored by PCRE, since it can match only at the start of
111			a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
112			optimization, because the . metacharacter does not then match a newline, and if
113			the subject string contains newlines, the pattern may match from the character
114			immediately following one of them instead of from the very start. For example,
115			the pattern
116	nigel	75	.sp
117	nigel	63	.*second
118	nigel	75	.sp
119			matches the subject "first\enand second" (where \en stands for a newline
120	nigel	63	character), with the match starting at the seventh character. In order to do
121			this, PCRE has to retry the match starting after every newline in the subject.
122	nigel	75	.P
123	nigel	63	If you are using such a pattern with subject strings that do not contain
124			newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
125	nigel	77	the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
126			from having to scan along the subject looking for a newline to restart at.
127	nigel	75	.P
128	nigel	63	Beware of patterns that contain nested indefinite repeats. These can take a
129			long time to run when applied to a string that does not match. Consider the
130			pattern fragment
131	nigel	75	.sp
132	nigel	93	^(a+)*
133	nigel	75	.sp
134	nigel	93	This can match "aaaa" in 16 different ways, and this number increases very
135	nigel	63	rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
136	nigel	93	times, and for each of those cases other than 0 or 4, the + repeats can match
137	nigel	63	different numbers of times.) When the remainder of the pattern is such that the
138			entire match is going to fail, PCRE has in principle to try every possible
139	nigel	93	variation, and this can take an extremely long time, even for relatively short
140			strings.
141	nigel	75	.P
142	nigel	63	An optimization catches some of the more simple cases such as
143	nigel	75	.sp
144	nigel	63	(a+)*b
145	nigel	75	.sp
146	nigel	63	where a literal character follows. Before embarking on the standard matching
147			procedure, PCRE checks that there is a "b" later in the subject string, and if
148			there is not, it fails the match immediately. However, when there is no
149			following literal this optimization cannot be used. You can see the difference
150			by comparing the behaviour of
151	nigel	75	.sp
152			(a+)*\ed
153			.sp
154	nigel	63	with the pattern above. The former gives a failure almost instantly when
155			applied to a whole line of "a" characters, whereas the latter takes an
156			appreciable time with strings longer than about 20 characters.
157	nigel	75	.P
158			In many cases, the solution to this kind of performance issue is to use an
159			atomic group or a possessive quantifier.
160	ph10	99	.
161			.
162			.SH AUTHOR
163			.rs
164			.sp
165			.nf
166			Philip Hazel
167			University Computing Service
168			Cambridge CB2 3QH, England.
169			.fi
170			.
171			.
172			.SH REVISION
173			.rs
174			.sp
175			.nf
176	ph10	859	Last updated: 09 January 2012
177			Copyright (c) 1997-2012 University of Cambridge.
178	ph10	99	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"