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       PCRE - Perl-compatible regular expressions


       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.


       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


       is compiled as if it were


       (Technical aside: It is done this way so that backtrack points within each of the  repeti-
       tions 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


       uses 51K bytes when compiled. When PCRE is compiled with its default internal pointer size
       of two bytes, the size limit on a compiled pattern 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 "subrou-
       tine" facility. Re-writing the above pattern as


       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.


       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 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 sub-
       ject 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 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 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


       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 prin-
       ciple  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


       where a literal character follows. Before embarking on the  standard  matching  procedure,
       PCRE 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  can-
       not 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.

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


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


       Last updated: 06 March 2007
       Copyright (c) 1997-2007 University of Cambridge.

       See attributes(5) for descriptions of the following attributes:

       |Availability	    | SUNWpcre	      |
       |Interface Stability | Uncommitted     |
       Source for PCRE is available on http://opensolaris.org.

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