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 processing time. The way you express your pattern as a regular expression
       can affect both of them.

MEMORY USAGE


       Patterns are compiled by PCRE into a reasonably efficient byte code, so that most simple patterns do not use much memory. However, there is
       one  case  where  memory  usage	can  be unexpectedly large. When 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
       particularly 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. When PCRE is compiled with its default internal pointer size of two bytes, the size limit on a compiled pat-
       tern is 64K, 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.

       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 pat-
       tern  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 accept-
       able, this kind of rewriting will allow you to process patterns that PCRE cannot otherwise handle.

PROCESSING TIME


       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 expres-
       sions for efficient performance. This document contains a few observations about PCRE.

       Using  Unicode  character properties (the p, P, and X 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.

       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  pat-
       tern

	 .*second

       matches	the  subject  "first
and  second" (where 
 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 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.

       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
       extremely 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. However, 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: 06 March 2007
       Copyright (c) 1997-2007 University of Cambridge.

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