• Last 5 Forum Topics
    Last post

The Web Only This Site



  • MARC

    Mailing list ARChives
    - Search by -


    Computing Dictionary

  • Text Link Ads

  • LINUX man pages
  • Linux Man Page Viewer

    The following form allows you to view linux man pages.





           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.


           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 lim-
           ited  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 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
           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
           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
           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
           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
           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
           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


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

  • Linux

    The Distributions


    The Software


    The News


  • Toll Free
Copyright © 1999 - 2016 by LinuxGuruz