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 <html>
 <head>
 <title>pcre2perform specification</title>
 </head>
 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
 <h1>pcre2perform man page</h1>
 <p>
 Return to the <a href="index.html">PCRE2 index page</a>.
 </p>
 <p>
 This page is part of the PCRE2 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>
 <ul>
 <li><a name="TOC1" href="#SEC1">PCRE2 PERFORMANCE</a>
 <li><a name="TOC2" href="#SEC2">COMPILED PATTERN MEMORY USAGE</a>
 <li><a name="TOC3" href="#SEC3">STACK USAGE AT RUN TIME</a>
 <li><a name="TOC4" href="#SEC4">PROCESSING TIME</a>
 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
 <li><a name="TOC6" href="#SEC6">REVISION</a>
 </ul>
 <br><a name="SEC1" href="#TOC1">PCRE2 PERFORMANCE</a><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><a name="SEC2" href="#TOC1">COMPILED PATTERN MEMORY USAGE</a><br>
 <P>
 Patterns are compiled by PCRE2 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 PCRE2 is compiled
 with its default internal pointer size of two bytes, the size limit on a
 compiled pattern is 64K code units in the 8-bit and 16-bit libraries, and this
 is reached with the above pattern if the outer repetition is increased from 3
 to 4. PCRE2 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
 PCRE2's
 <a href="pcre2pattern.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="pcre2pattern.html#atomicgroup">atomic groups</a>
 into which there can be no backtracking if there is a subsequent matching
 failure. Therefore, PCRE2 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 PCRE2 cannot otherwise handle.
 </P>
 <br><a name="SEC3" href="#TOC1">STACK USAGE AT RUN TIME</a><br>
 <P>
 When <b>pcre2_match()</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. Rewriting your pattern can often help. The
 <a href="pcre2stack.html"><b>pcre2stack</b></a>
 documentation discusses this issue in detail.
 </P>
 <br><a name="SEC4" href="#TOC1">PROCESSING TIME</a><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 PCRE2.
 </P>
 <P>
 Using Unicode character properties (the \p, \P, and \X escapes) is slow,
 because PCRE2 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 the PCRE2_UCP option or start the pattern with (*UCP) if you want Unicode
 character properties to be used. This can double the matching time for items
 such as \d, when matched with <b>pcre2_match()</b>; 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 atomic parentheses, nor in parentheses
 that are the subject of a backreference, and the PCRE2_DOTALL option is set,
 the pattern is implicitly anchored by PCRE2, since it can match only at the
 start of a subject string. If the pattern has multiple top-level branches, they
 must all be anchorable. The optimization can be disabled by the
 PCRE2_NO_DOTSTAR_ANCHOR option, and is automatically disabled if the pattern
 contains (*PRUNE) or (*SKIP).
 </P>
 <P>
 If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, because the
 dot 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, PCRE2 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 PCRE2_DOTALL, or starting
 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE2
 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, PCRE2 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, PCRE2 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><a name="SEC5" href="#TOC1">AUTHOR</a><br>
 <P>
 Philip Hazel
 <br>
 University Computing Service
 <br>
 Cambridge, England.
 <br>
 </P>
 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
 <P>
 Last updated: 02 January 2015
 <br>
 Copyright &copy; 1997-2015 University of Cambridge.
 <br>
 <p>
 Return to the <a href="index.html">PCRE2 index page</a>.
 </p>
	<html>
	<head>
	<title>pcre2perform specification</title>
	</head>
	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
	<h1>pcre2perform man page</h1>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>
	<p>
	This page is part of the PCRE2 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>
	<ul>
	<li><a name="TOC1" href="#SEC1">PCRE2 PERFORMANCE</a>
	<li><a name="TOC2" href="#SEC2">COMPILED PATTERN MEMORY USAGE</a>
	<li><a name="TOC3" href="#SEC3">STACK USAGE AT RUN TIME</a>
	<li><a name="TOC4" href="#SEC4">PROCESSING TIME</a>
	<li><a name="TOC5" href="#SEC5">AUTHOR</a>
	<li><a name="TOC6" href="#SEC6">REVISION</a>
	</ul>
	<br><a name="SEC1" href="#TOC1">PCRE2 PERFORMANCE</a><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><a name="SEC2" href="#TOC1">COMPILED PATTERN MEMORY USAGE</a><br>
	<P>
	Patterns are compiled by PCRE2 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 PCRE2 is compiled
	with its default internal pointer size of two bytes, the size limit on a
	compiled pattern is 64K code units in the 8-bit and 16-bit libraries, and this
	is reached with the above pattern if the outer repetition is increased from 3
	to 4. PCRE2 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
	PCRE2's
	<a href="pcre2pattern.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="pcre2pattern.html#atomicgroup">atomic groups</a>
	into which there can be no backtracking if there is a subsequent matching
	failure. Therefore, PCRE2 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 PCRE2 cannot otherwise handle.
	</P>
	<br><a name="SEC3" href="#TOC1">STACK USAGE AT RUN TIME</a><br>
	<P>
	When <b>pcre2_match()</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. Rewriting your pattern can often help. The
	<a href="pcre2stack.html"><b>pcre2stack</b></a>
	documentation discusses this issue in detail.
	</P>
	<br><a name="SEC4" href="#TOC1">PROCESSING TIME</a><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 PCRE2.
	</P>
	<P>
	Using Unicode character properties (the \p, \P, and \X escapes) is slow,
	because PCRE2 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 the PCRE2_UCP option or start the pattern with (*UCP) if you want Unicode
	character properties to be used. This can double the matching time for items
	such as \d, when matched with <b>pcre2_match()</b>; 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 atomic parentheses, nor in parentheses
	that are the subject of a backreference, and the PCRE2_DOTALL option is set,
	the pattern is implicitly anchored by PCRE2, since it can match only at the
	start of a subject string. If the pattern has multiple top-level branches, they
	must all be anchorable. The optimization can be disabled by the
	PCRE2_NO_DOTSTAR_ANCHOR option, and is automatically disabled if the pattern
	contains (PRUNE) or (SKIP).
	</P>
	<P>
	If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, because the
	dot 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, PCRE2 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 PCRE2_DOTALL, or starting
	the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE2
	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, PCRE2 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, PCRE2 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><a name="SEC5" href="#TOC1">AUTHOR</a><br>
	<P>
	Philip Hazel
	<br>
	University Computing Service
	<br>
	Cambridge, England.
	<br>
	</P>
	<br><a name="SEC6" href="#TOC1">REVISION</a><br>
	<P>
	Last updated: 02 January 2015
	<br>
	Copyright © 1997-2015 University of Cambridge.
	<br>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>