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PCREPERFORM(3)		   Library Functions Manual		PCREPERFORM(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE PERFORMANCE
       Two  aspects  of	performance are	discussed below: memory	usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.

COMPILED PATTERN MEMORY	USAGE
       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

	 (abc|def){2,4}

       is compiled as if it were

	 (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so	that backtrack	points	within
       each of the repetitions can be independently maintained.)

       For  regular expressions	whose quantifiers use only small numbers, this
       is not usually a	problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can	become
       an embarrassment. For example, the very simple pattern

	 ((ab){1,1000}c){1,3}

       uses 51K	bytes when compiled using the 8-bit library. When PCRE is com-
       piled  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 pat-
       tern to use less	memory if you can.

       One way of reducing the memory usage for	such patterns is to  make  use
       of PCRE's "subroutine" facility.	Re-writing the above pattern as

	 ((ab)(?2){0,999}c)(?1){0,2}

       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 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.

STACK USAGE AT RUN TIME
       When pcre_exec()	or pcre[16|32]_exec() 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 pcrestack documentation discusses this issue	in de-
       tail.

PROCESSING TIME
       Certain items in	regular	expression patterns are	processed  more	 effi-
       ciently 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 ob-
       servations about	PCRE.

       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.

       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.

       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  new-
       lines,  the  pattern may	match from the character immediately following
       one of them instead of from the very start. For example,	the pattern

	 .*second

       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.

       If  you	are using such a pattern with subject strings that do not con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or  starting  the pattern with ^.* or ^.*? to indicate explicit anchor-
       ing. That saves PCRE from having	to scan	along the subject looking  for
       a newline to restart at.

       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

	 ^(a+)*

       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  ex-
       tremely long time, even for relatively short strings.

       An optimization catches some of the more	simple cases such as

	 (a+)*b

       where  a	 literal  character  follows. Before embarking on the standard
       matching	procedure, PCRE	checks that there is a "b" later in  the  sub-
       ject  string, and if there is not, it fails the match immediately. How-
       ever, when there	is no following	literal	this  optimization  cannot  be
       used. You can see the difference	by comparing the behaviour of

	 (a+)*\d

       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.

       In many cases, the solution to this kind	of performance issue is	to use
       an atomic group or a possessive quantifier.

AUTHOR
       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION
       Last updated: 25	August 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30			09 January 2012			PCREPERFORM(3)

NAME | PCRE PERFORMANCE | COMPILED PATTERN MEMORY USAGE | STACK USAGE AT RUN TIME | PROCESSING TIME | AUTHOR | REVISION

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