1 .TH PCREPERFORM 3

     2 .SH NAME

     3 PCRE - Perl-compatible regular expressions

     4 .SH "PCRE PERFORMANCE"

     5 .rs

     6 .sp

     7 Two aspects of performance are discussed below: memory usage and processing

     8 time. The way you express your pattern as a regular expression can affect both

     9 of them.

    10 .

    11 .SH "MEMORY USAGE"

    12 .rs

    13 .sp

    14 Patterns are compiled by PCRE into a reasonably efficient byte code, so that

    15 most simple patterns do not use much memory. However, there is one case where

    16 memory usage can be unexpectedly large. When a parenthesized subpattern has a

    17 quantifier with a minimum greater than 1 and/or a limited maximum, the whole

    18 subpattern is repeated in the compiled code. For example, the pattern

    19 .sp

    20   (abc|def){2,4}

    21 .sp

    22 is compiled as if it were

    23 .sp

    24   (abc|def)(abc|def)((abc|def)(abc|def)?)?

    25 .sp

    26 (Technical aside: It is done this way so that backtrack points within each of

    27 the repetitions can be independently maintained.)

    28 .P

    29 For regular expressions whose quantifiers use only small numbers, this is not

    30 usually a problem. However, if the numbers are large, and particularly if such

    31 repetitions are nested, the memory usage can become an embarrassment. For

    32 example, the very simple pattern

    33 .sp

    34   ((ab){1,1000}c){1,3}

    35 .sp

    36 uses 51K bytes when compiled. When PCRE is compiled with its default internal

    37 pointer size of two bytes, the size limit on a compiled pattern is 64K, and

    38 this is reached with the above pattern if the outer repetition is increased

    39 from 3 to 4. PCRE can be compiled to use larger internal pointers and thus

    40 handle larger compiled patterns, but it is better to try to rewrite your

    41 pattern to use less memory if you can.

    42 .P

    43 One way of reducing the memory usage for such patterns is to make use of PCRE's

    44 .\" HTML <a href="pcrepattern.html#subpatternsassubroutines">

    45 .\" </a>

    46 "subroutine"

    47 .\"

    48 facility. Re-writing the above pattern as

    49 .sp

    50   ((ab)(?2){0,999}c)(?1){0,2}

    51 .sp

    52 reduces the memory requirements to 18K, and indeed it remains under 20K even

    53 with the outer repetition increased to 100. However, this pattern is not

    54 exactly equivalent, because the "subroutine" calls are treated as

    55 .\" HTML <a href="pcrepattern.html#atomicgroup">

    56 .\" </a>

    57 atomic groups

    58 .\"

    59 into which there can be no backtracking if there is a subsequent matching

    60 failure. Therefore, PCRE cannot do this kind of rewriting automatically.

    61 Furthermore, there is a noticeable loss of speed when executing the modified

    62 pattern. Nevertheless, if the atomic grouping is not a problem and the loss of

    63 speed is acceptable, this kind of rewriting will allow you to process patterns

    64 that PCRE cannot otherwise handle.

    65 .

    66 .SH "PROCESSING TIME"

    67 .rs

    68 .sp

    69 Certain items in regular expression patterns are processed more efficiently

    70 than others. It is more efficient to use a character class like [aeiou] than a

    71 set of single-character alternatives such as (a|e|i|o|u). In general, the

    72 simplest construction that provides the required behaviour is usually the most

    73 efficient. Jeffrey Friedl's book contains a lot of useful general discussion

    74 about optimizing regular expressions for efficient performance. This document

    75 contains a few observations about PCRE.

    76 .P

    77 Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,

    78 because PCRE has to scan a structure that contains data for over fifteen

    79 thousand characters whenever it needs a character's property. If you can find

    80 an alternative pattern that does not use character properties, it will probably

    81 be faster.

    82 .P

    83 When a pattern begins with .* not in parentheses, or in parentheses that are

    84 not the subject of a backreference, and the PCRE_DOTALL option is set, the

    85 pattern is implicitly anchored by PCRE, since it can match only at the start of

    86 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this

    87 optimization, because the . metacharacter does not then match a newline, and if

    88 the subject string contains newlines, the pattern may match from the character

    89 immediately following one of them instead of from the very start. For example,

    90 the pattern

    91 .sp

    92   .*second

    93 .sp

    94 matches the subject "first\enand second" (where \en stands for a newline

    95 character), with the match starting at the seventh character. In order to do

    96 this, PCRE has to retry the match starting after every newline in the subject.

    97 .P

    98 If you are using such a pattern with subject strings that do not contain

    99 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting

   100 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE

   101 from having to scan along the subject looking for a newline to restart at.

   102 .P

   103 Beware of patterns that contain nested indefinite repeats. These can take a

   104 long time to run when applied to a string that does not match. Consider the

   105 pattern fragment

   106 .sp

   107   ^(a+)*

   108 .sp

   109 This can match "aaaa" in 16 different ways, and this number increases very

   110 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4

   111 times, and for each of those cases other than 0 or 4, the + repeats can match

   112 different numbers of times.) When the remainder of the pattern is such that the

   113 entire match is going to fail, PCRE has in principle to try every possible

   114 variation, and this can take an extremely long time, even for relatively short

   115 strings.

   116 .P

   117 An optimization catches some of the more simple cases such as

   118 .sp

   119   (a+)*b

   120 .sp

   121 where a literal character follows. Before embarking on the standard matching

   122 procedure, PCRE checks that there is a "b" later in the subject string, and if

   123 there is not, it fails the match immediately. However, when there is no

   124 following literal this optimization cannot be used. You can see the difference

   125 by comparing the behaviour of

   126 .sp

   127   (a+)*\ed

   128 .sp

   129 with the pattern above. The former gives a failure almost instantly when

   130 applied to a whole line of "a" characters, whereas the latter takes an

   131 appreciable time with strings longer than about 20 characters.

   132 .P

   133 In many cases, the solution to this kind of performance issue is to use an

   134 atomic group or a possessive quantifier.

   135 .

   136 .

   137 .SH AUTHOR

   138 .rs

   139 .sp

   140 .nf

   141 Philip Hazel

   142 University Computing Service

   143 Cambridge CB2 3QH, England.

   144 .fi

   145 .

   146 .

   147 .SH REVISION

   148 .rs

   149 .sp

   150 .nf

   151 Last updated: 06 March 2007

   152 Copyright (c) 1997-2007 University of Cambridge.

   153 .fi