doc/html/pcreperform.html

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<title>pcreperform specification</title>
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<h1>pcreperform man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
<br>
<br><b>
PCRE PERFORMANCE
</b><br>
<P>
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.
</P>
<br><b>
COMPILED PATTERN MEMORY USAGE
</b><br>
<P>
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
<pre>
  (abc|def){2,4}
</pre>
is compiled as if it were
<pre>
  (abc|def)(abc|def)((abc|def)(abc|def)?)?
</pre>
(Technical aside: It is done this way so that backtrack points within each of
the repetitions can be independently maintained.)
</P>
<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
<pre>
  ((ab){1,1000}c){1,3}
</pre>
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>
<P>
One way of reducing the memory usage for such patterns is to make use of PCRE's
<a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
facility. Re-writing the above pattern as
<pre>
  ((ab)(?2){0,999}c)(?1){0,2}
</pre>
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
<a href="pcrepattern.html#atomicgroup">atomic groups</a>
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.
</P>
<br><b>
STACK USAGE AT RUN TIME
</b><br>
<P>
When <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> 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
<a href="pcrestack.html"><b>pcrestack</b></a>
documentation discusses this issue in detail.
</P>
<br><b>
PROCESSING TIME
</b><br>
<P>
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>
<P>
Using Unicode character properties (the \p, \P, and \X escapes) is slow,
because PCRE has to use a multi-stage table lookup 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>
<P>
By default, the escape sequences \b, \d, \s, and \w, 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 \d, 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
\b.
</P>
<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
<pre>
  .*second
</pre>
matches the subject "first\nand second" (where \n 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>
<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>
<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
<pre>
  ^(a+)*
</pre>
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>
<P>
An optimization catches some of the more simple cases such as
<pre>
  (a+)*b
</pre>
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
<pre>
  (a+)*\d
</pre>
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>
<P>
In many cases, the solution to this kind of performance issue is to use an
atomic group or a possessive quantifier.
</P>
<br><b>
AUTHOR
</b><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><b>
REVISION
</b><br>
<P>
Last updated: 25 August 2012
<br>
Copyright &copy; 1997-2012 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	<html>
2	<head>
3	<title>pcreperform specification</title>
4	</head>
5	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	<h1>pcreperform man page</h1>
7	<p>
8	Return to the <a href="index.html">PCRE index page</a>.
9	</p>
10	<p>
11	This page is part of the PCRE HTML documentation. It was generated automatically
12	from the original man page. If there is any nonsense in it, please consult the
13	man page, in case the conversion went wrong.
14	<br>
15	<br><b>
16	PCRE PERFORMANCE
17	</b><br>
18	<P>
19	Two aspects of performance are discussed below: memory usage and processing
20	time. The way you express your pattern as a regular expression can affect both
21	of them.
22	</P>
23	<br><b>
24	COMPILED PATTERN MEMORY USAGE
25	</b><br>
26	<P>
27	Patterns are compiled by PCRE into a reasonably efficient interpretive code, so
28	that most simple patterns do not use much memory. However, there is one case
29	where the memory usage of a compiled pattern can be unexpectedly large. If a
30	parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
31	a limited maximum, the whole subpattern is repeated in the compiled code. For
32	example, the pattern
33	<pre>
34	(abc\|def){2,4}
35	</pre>
36	is compiled as if it were
37	<pre>
38	(abc\|def)(abc\|def)((abc\|def)(abc\|def)?)?
39	</pre>
40	(Technical aside: It is done this way so that backtrack points within each of
41	the repetitions can be independently maintained.)
42	</P>
43	<P>
44	For regular expressions whose quantifiers use only small numbers, this is not
45	usually a problem. However, if the numbers are large, and particularly if such
46	repetitions are nested, the memory usage can become an embarrassment. For
47	example, the very simple pattern
48	<pre>
49	((ab){1,1000}c){1,3}
50	</pre>
51	uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
52	with its default internal pointer size of two bytes, the size limit on a
53	compiled pattern is 64K data units, and this is reached with the above pattern
54	if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
55	larger internal pointers and thus handle larger compiled patterns, but it is
56	better to try to rewrite your pattern to use less memory if you can.
57	</P>
58	<P>
59	One way of reducing the memory usage for such patterns is to make use of PCRE's
60	<a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
61	facility. Re-writing the above pattern as
62	<pre>
63	((ab)(?2){0,999}c)(?1){0,2}
64	</pre>
65	reduces the memory requirements to 18K, and indeed it remains under 20K even
66	with the outer repetition increased to 100. However, this pattern is not
67	exactly equivalent, because the "subroutine" calls are treated as
68	<a href="pcrepattern.html#atomicgroup">atomic groups</a>
69	into which there can be no backtracking if there is a subsequent matching
70	failure. Therefore, PCRE cannot do this kind of rewriting automatically.
71	Furthermore, there is a noticeable loss of speed when executing the modified
72	pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
73	speed is acceptable, this kind of rewriting will allow you to process patterns
74	that PCRE cannot otherwise handle.
75	</P>
76	<br><b>
77	STACK USAGE AT RUN TIME
78	</b><br>
79	<P>
80	When <b>pcre_exec()</b> or <b>pcre[16\|32]_exec()</b> is used for matching, certain
81	kinds of pattern can cause it to use large amounts of the process stack. In
82	some environments the default process stack is quite small, and if it runs out
83	the result is often SIGSEGV. This issue is probably the most frequently raised
84	problem with PCRE. Rewriting your pattern can often help. The
85	<a href="pcrestack.html"><b>pcrestack</b></a>
86	documentation discusses this issue in detail.
87	</P>
88	<br><b>
89	PROCESSING TIME
90	</b><br>
91	<P>
92	Certain items in regular expression patterns are processed more efficiently
93	than others. It is more efficient to use a character class like [aeiou] than a
94	set of single-character alternatives such as (a\|e\|i\|o\|u). In general, the
95	simplest construction that provides the required behaviour is usually the most
96	efficient. Jeffrey Friedl's book contains a lot of useful general discussion
97	about optimizing regular expressions for efficient performance. This document
98	contains a few observations about PCRE.
99	</P>
100	<P>
101	Using Unicode character properties (the \p, \P, and \X escapes) is slow,
102	because PCRE has to use a multi-stage table lookup whenever it needs a
103	character's property. If you can find an alternative pattern that does not use
104	character properties, it will probably be faster.
105	</P>
106	<P>
107	By default, the escape sequences \b, \d, \s, and \w, and the POSIX
108	character classes such as [:alpha:] do not use Unicode properties, partly for
109	backwards compatibility, and partly for performance reasons. However, you can
110	set PCRE_UCP if you want Unicode character properties to be used. This can
111	double the matching time for items such as \d, when matched with
112	a traditional matching function; the performance loss is less with
113	a DFA matching function, and in both cases there is not much difference for
114	\b.
115	</P>
116	<P>
117	When a pattern begins with .* not in parentheses, or in parentheses that are
118	not the subject of a backreference, and the PCRE_DOTALL option is set, the
119	pattern is implicitly anchored by PCRE, since it can match only at the start of
120	a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
121	optimization, because the . metacharacter does not then match a newline, and if
122	the subject string contains newlines, the pattern may match from the character
123	immediately following one of them instead of from the very start. For example,
124	the pattern
125	<pre>
126	.*second
127	</pre>
128	matches the subject "first\nand second" (where \n stands for a newline
129	character), with the match starting at the seventh character. In order to do
130	this, PCRE has to retry the match starting after every newline in the subject.
131	</P>
132	<P>
133	If you are using such a pattern with subject strings that do not contain
134	newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
135	the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
136	from having to scan along the subject looking for a newline to restart at.
137	</P>
138	<P>
139	Beware of patterns that contain nested indefinite repeats. These can take a
140	long time to run when applied to a string that does not match. Consider the
141	pattern fragment
142	<pre>
143	^(a+)*
144	</pre>
145	This can match "aaaa" in 16 different ways, and this number increases very
146	rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
147	times, and for each of those cases other than 0 or 4, the + repeats can match
148	different numbers of times.) When the remainder of the pattern is such that the
149	entire match is going to fail, PCRE has in principle to try every possible
150	variation, and this can take an extremely long time, even for relatively short
151	strings.
152	</P>
153	<P>
154	An optimization catches some of the more simple cases such as
155	<pre>
156	(a+)*b
157	</pre>
158	where a literal character follows. Before embarking on the standard matching
159	procedure, PCRE checks that there is a "b" later in the subject string, and if
160	there is not, it fails the match immediately. However, when there is no
161	following literal this optimization cannot be used. You can see the difference
162	by comparing the behaviour of
163	<pre>
164	(a+)*\d
165	</pre>
166	with the pattern above. The former gives a failure almost instantly when
167	applied to a whole line of "a" characters, whereas the latter takes an
168	appreciable time with strings longer than about 20 characters.
169	</P>
170	<P>
171	In many cases, the solution to this kind of performance issue is to use an
172	atomic group or a possessive quantifier.
173	</P>
174	<br><b>
175	AUTHOR
176	</b><br>
177	<P>
178	Philip Hazel
179	<br>
180	University Computing Service
181	<br>
182	Cambridge CB2 3QH, England.
183	<br>
184	</P>
185	<br><b>
186	REVISION
187	</b><br>
188	<P>
189	Last updated: 25 August 2012
190	<br>
191	Copyright © 1997-2012 University of Cambridge.
192	<br>
193	<p>
194	Return to the <a href="index.html">PCRE index page</a>.
195	</p>
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"