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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 AND HEAP 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 for storing the compiled
 version. However, there is one case where the memory usage of a compiled
 pattern can be unexpectedly large. If a parenthesized group has a quantifier
 with a minimum greater than 1 and/or a limited maximum, the whole group 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 over 50KiB 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 65535 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 around 16KiB, and indeed it remains under
 20KiB even with the outer repetition increased to 100. However, this kind of
 pattern is not always exactly equivalent, because any captures within
 subroutine calls are lost when the subroutine completes. If this is not a
 problem, this kind of rewriting will allow you to process patterns that PCRE2
 cannot otherwise handle. The matching performance of the two different versions
 of the pattern are roughly the same. (This applies from release 10.30 - things
 were different in earlier releases.)
 </P>
 <br><a name="SEC3" href="#TOC1">STACK AND HEAP USAGE AT RUN TIME</a><br>
 <P>
 From release 10.30, the interpretive (non-JIT) version of <b>pcre2_match()</b>
 uses very little system stack at run time. In earlier releases recursive
 function calls could use a great deal of stack, and this could cause problems,
 but this usage has been eliminated. Backtracking positions are now explicitly
 remembered in memory frames controlled by the code.
 </P>
 <P>
 The size of each frame depends on the size of pointer variables and the number
 of capturing parenthesized groups in the pattern being matched. On a 64-bit
 system the frame size for a pattern with no captures is 128 bytes. For each
 capturing group the size increases by 16 bytes.
 </P>
 <P>
 Until release 10.41, an initial 20KiB frames vector was allocated on the system
 stack, but this still caused some issues for multi-thread applications where
 each thread has a very small stack. From release 10.41 backtracking memory
 frames are always held in heap memory. An initial heap allocation is obtained
 the first time any match data block is passed to <b>pcre2_match()</b>. This is
 remembered with the match data block and re-used if that block is used for
 another match. It is freed when the match data block itself is freed.
 </P>
 <P>
 The size of the initial block is the larger of 20KiB or ten times the pattern's
 frame size, unless the heap limit is less than this, in which case the heap
 limit is used. If the initial block proves to be too small during matching, it
 is replaced by a larger block, subject to the heap limit. The heap limit is
 checked only when a new block is to be allocated. Reducing the heap limit
 between calls to <b>pcre2_match()</b> with the same match data block does not
 affect the saved block.
 </P>
 <P>
 In contrast to <b>pcre2_match()</b>, <b>pcre2_dfa_match()</b> does use recursive
 function calls, but only for processing atomic groups, lookaround assertions,
 and recursion within the pattern. The original version of the code used to
 allocate quite large internal workspace vectors on the stack, which caused some
 problems for some patterns in environments with small stacks. From release
 10.32 the code for <b>pcre2_dfa_match()</b> has been re-factored to use heap
 memory when necessary for internal workspace when recursing, though recursive
 function calls are still used.
 </P>
 <P>
 The "match depth" parameter can be used to limit the depth of function
 recursion, and the "match heap" parameter to limit heap memory in
 <b>pcre2_dfa_match()</b>.
 </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. This can often reduce memory
 requirements as well. As another example, consider this pattern:
 <pre>
   ([^&#60;]|&#60;(?!inet))+
 </pre>
 It matches from wherever it starts until it encounters "&#60;inet" or the end of
 the data, and is the kind of pattern that might be used when processing an XML
 file. Each iteration of the outer parentheses matches either one character that
 is not "&#60;" or a "&#60;" that is not followed by "inet". However, each time a
 parenthesis is processed, a backtracking position is passed, so this
 formulation uses a memory frame for each matched character. For a long string,
 a lot of memory is required. Consider now this rewritten pattern, which matches
 exactly the same strings:
 <pre>
   ([^&#60;]++|&#60;(?!inet))+
 </pre>
 This runs much faster, because sequences of characters that do not contain "&#60;"
 are "swallowed" in one item inside the parentheses, and a possessive quantifier
 is used to stop any backtracking into the runs of non-"&#60;" characters. This
 version also uses a lot less memory because entry to a new set of parentheses
 happens only when a "&#60;" character that is not followed by "inet" is encountered
 (and we assume this is relatively rare).
 </P>
 <P>
 This example shows that one way of optimizing performance when matching long
 subject strings is to write repeated parenthesized subpatterns to match more
 than one character whenever possible.
 </P>
 <br><b>
 SETTING RESOURCE LIMITS
 </b><br>
 <P>
 You can set limits on the amount of processing that takes place when matching,
 and on the amount of heap memory that is used. The default values of the limits
 are very large, and unlikely ever to operate. They can be changed when PCRE2 is
 built, and they can also be set when <b>pcre2_match()</b> or
 <b>pcre2_dfa_match()</b> is called. For details of these interfaces, see the
 <a href="pcre2build.html"><b>pcre2build</b></a>
 documentation and the section entitled
 <a href="pcre2api.html#matchcontext">"The match context"</a>
 in the
 <a href="pcre2api.html"><b>pcre2api</b></a>
 documentation.
 </P>
 <P>
 The <b>pcre2test</b> test program has a modifier called "find_limits" which, if
 applied to a subject line, causes it to find the smallest limits that allow a
 pattern to match. This is done by repeatedly matching with different limits.
 </P>
 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
 <P>
 Philip Hazel
 <br>
 Retired from University Computing Service
 <br>
 Cambridge, England.
 <br>
 </P>
 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
 <P>
 Last updated: 27 July 2022
 <br>
 Copyright &copy; 1997-2022 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 AND HEAP 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 for storing the compiled
	version. However, there is one case where the memory usage of a compiled
	pattern can be unexpectedly large. If a parenthesized group has a quantifier
	with a minimum greater than 1 and/or a limited maximum, the whole group 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 over 50KiB 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 65535 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 around 16KiB, and indeed it remains under
	20KiB even with the outer repetition increased to 100. However, this kind of
	pattern is not always exactly equivalent, because any captures within
	subroutine calls are lost when the subroutine completes. If this is not a
	problem, this kind of rewriting will allow you to process patterns that PCRE2
	cannot otherwise handle. The matching performance of the two different versions
	of the pattern are roughly the same. (This applies from release 10.30 - things
	were different in earlier releases.)
	</P>
	<br><a name="SEC3" href="#TOC1">STACK AND HEAP USAGE AT RUN TIME</a><br>
	<P>
	From release 10.30, the interpretive (non-JIT) version of <b>pcre2_match()</b>
	uses very little system stack at run time. In earlier releases recursive
	function calls could use a great deal of stack, and this could cause problems,
	but this usage has been eliminated. Backtracking positions are now explicitly
	remembered in memory frames controlled by the code.
	</P>
	<P>
	The size of each frame depends on the size of pointer variables and the number
	of capturing parenthesized groups in the pattern being matched. On a 64-bit
	system the frame size for a pattern with no captures is 128 bytes. For each
	capturing group the size increases by 16 bytes.
	</P>
	<P>
	Until release 10.41, an initial 20KiB frames vector was allocated on the system
	stack, but this still caused some issues for multi-thread applications where
	each thread has a very small stack. From release 10.41 backtracking memory
	frames are always held in heap memory. An initial heap allocation is obtained
	the first time any match data block is passed to <b>pcre2_match()</b>. This is
	remembered with the match data block and re-used if that block is used for
	another match. It is freed when the match data block itself is freed.
	</P>
	<P>
	The size of the initial block is the larger of 20KiB or ten times the pattern's
	frame size, unless the heap limit is less than this, in which case the heap
	limit is used. If the initial block proves to be too small during matching, it
	is replaced by a larger block, subject to the heap limit. The heap limit is
	checked only when a new block is to be allocated. Reducing the heap limit
	between calls to <b>pcre2_match()</b> with the same match data block does not
	affect the saved block.
	</P>
	<P>
	In contrast to <b>pcre2_match()</b>, <b>pcre2_dfa_match()</b> does use recursive
	function calls, but only for processing atomic groups, lookaround assertions,
	and recursion within the pattern. The original version of the code used to
	allocate quite large internal workspace vectors on the stack, which caused some
	problems for some patterns in environments with small stacks. From release
	10.32 the code for <b>pcre2_dfa_match()</b> has been re-factored to use heap
	memory when necessary for internal workspace when recursing, though recursive
	function calls are still used.
	</P>
	<P>
	The "match depth" parameter can be used to limit the depth of function
	recursion, and the "match heap" parameter to limit heap memory in
	<b>pcre2_dfa_match()</b>.
	</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. This can often reduce memory
	requirements as well. As another example, consider this pattern:
	<pre>
	([^<]\|<(?!inet))+
	</pre>
	It matches from wherever it starts until it encounters "<inet" or the end of
	the data, and is the kind of pattern that might be used when processing an XML
	file. Each iteration of the outer parentheses matches either one character that
	is not "<" or a "<" that is not followed by "inet". However, each time a
	parenthesis is processed, a backtracking position is passed, so this
	formulation uses a memory frame for each matched character. For a long string,
	a lot of memory is required. Consider now this rewritten pattern, which matches
	exactly the same strings:
	<pre>
	([^<]++\|<(?!inet))+
	</pre>
	This runs much faster, because sequences of characters that do not contain "<"
	are "swallowed" in one item inside the parentheses, and a possessive quantifier
	is used to stop any backtracking into the runs of non-"<" characters. This
	version also uses a lot less memory because entry to a new set of parentheses
	happens only when a "<" character that is not followed by "inet" is encountered
	(and we assume this is relatively rare).
	</P>
	<P>
	This example shows that one way of optimizing performance when matching long
	subject strings is to write repeated parenthesized subpatterns to match more
	than one character whenever possible.
	</P>
	<br><b>
	SETTING RESOURCE LIMITS
	</b><br>
	<P>
	You can set limits on the amount of processing that takes place when matching,
	and on the amount of heap memory that is used. The default values of the limits
	are very large, and unlikely ever to operate. They can be changed when PCRE2 is
	built, and they can also be set when <b>pcre2_match()</b> or
	<b>pcre2_dfa_match()</b> is called. For details of these interfaces, see the
	<a href="pcre2build.html"><b>pcre2build</b></a>
	documentation and the section entitled
	<a href="pcre2api.html#matchcontext">"The match context"</a>
	in the
	<a href="pcre2api.html"><b>pcre2api</b></a>
	documentation.
	</P>
	<P>
	The <b>pcre2test</b> test program has a modifier called "find_limits" which, if
	applied to a subject line, causes it to find the smallest limits that allow a
	pattern to match. This is done by repeatedly matching with different limits.
	</P>
	<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
	<P>
	Philip Hazel
	<br>
	Retired from University Computing Service
	<br>
	Cambridge, England.
	<br>
	</P>
	<br><a name="SEC6" href="#TOC1">REVISION</a><br>
	<P>
	Last updated: 27 July 2022
	<br>
	Copyright © 1997-2022 University of Cambridge.
	<br>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>