/usr/man2/cat.3/pcreperform.3.Z(/usr/man2/cat.3/pcreperform.3.Z)

NAME

       PCRE - Perl-compatible regular expressions

PCRE PERFORMANCE

       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
       useful general discussion  about  optimizing  regular  expressions  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 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 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

         (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.

Last updated: 28 February 2005
Copyright (c) 1997-2005 University of Cambridge.

                                                                PCREPERFORM(3)