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.

COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE into a reasonably efficient interpretive code, so that most simple patterns
       do  not  use  much memory. However, there is one case where the memory usage of a compiled pattern can be
       unexpectedly large. If 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 using the 8-bit library. When PCRE is compiled  with  its  default  internal
       pointer  size  of  two bytes, the size limit on a compiled pattern is 64K data units, 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 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.

STACK USAGE AT RUN TIME

       When pcre_exec() or pcre[16|32]_exec() is used for matching, certain kinds of pattern can cause it to use
       large amounts of the process stack. In some environments the default process stack is quite small, and if
       it  runs  out the result is often SIGSEGV. This issue is probably the most frequently raised problem with
       PCRE. Rewriting your pattern can often help. The pcrestack documentation discusses this issue in detail.

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 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 use a multi-
       stage  table lookup 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.

       By default, the escape sequences \b, \d, \s, and \w, and the POSIX character classes such as [:alpha:] do
       not  use  Unicode  properties,  partly  for  backwards compatibility, and partly for performance reasons.
       However, you can set PCRE_UCP if you want Unicode character properties to be used. This  can  double  the
       matching  time  for  items such as \d, when matched with a traditional matching function; the performance
       loss is less with a DFA matching function, and in both cases there is not much difference for \b.

       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 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 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 subject 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: 25 August 2012
       Copyright (c) 1997-2012 University of Cambridge.