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PCRE(3)								       PCRE(3)

       PCRE - Perl-compatible regular expressions


       Certain	items  that may appear in regular expression patterns are more
       efficient than others. It is more efficient to use  a  character	 class
       like  [aeiou]  than  a set of alternatives such as (a|e|i|o|u). In gen‐
       eral, the simplest construction that provides the required behaviour is
       usually	the  most  efficient.  Jeffrey Friedl's book contains a lot of
       discussion about optimizing regular expressions for  efficient  perfor‐

       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


       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 ^.* 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


       This  can  match "aaaa" in 33 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, 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.

       An optimization catches some of the more simple cases such as


       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


       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.

Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.


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