update to pcre 7.9

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@13706 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/pcre/doc/html/pcreperform.html

@@ -16,13 +16,73 @@ man page, in case the conversion went wrong.
 PCRE PERFORMANCE
 </b><br>
 <P>
-Certain items that may appear in regular expression patterns are more efficient
+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.
+</P>
+<br><b>
+MEMORY USAGE
+</b><br>
+<P>
+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
+<pre>
+  (abc|def){2,4}
+</pre>
+is compiled as if it were
+<pre>
+  (abc|def)(abc|def)((abc|def)(abc|def)?)?
+</pre>
+(Technical aside: It is done this way so that backtrack points within each of
+the repetitions can be independently maintained.)
+</P>
+<P>
+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
+<pre>
+  ((ab){1,1000}c){1,3}
+</pre>
+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.
+</P>
+<P>
+One way of reducing the memory usage for such patterns is to make use of PCRE's
+<a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
+facility. Re-writing the above pattern as
+<pre>
+  ((ab)(?2){0,999}c)(?1){0,2}
+</pre>
+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
+<a href="pcrepattern.html#atomicgroup">atomic groups</a>
+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.
+</P>
+<br><b>
+PROCESSING TIME
+</b><br>
+<P>
+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 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.
+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.
 </P>
 <P>
 Using Unicode character properties (the \p, \P, and \X escapes) is slow,
@@ -58,14 +118,15 @@ 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
 <pre>
-  (a+)*
+  ^(a+)*
 </pre>
-This can match "aaaa" in 33 different ways, and this number increases very
+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, the + repeats can match
+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.
+variation, and this can take an extremely long time, even for relatively short
+strings.
 </P>
 <P>
 An optimization catches some of the more simple cases such as
@@ -88,10 +149,25 @@ 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.
 </P>
+<br><b>
+AUTHOR
+</b><br>
 <P>
-Last updated: 28 February 2005
+Philip Hazel
+<br>
+University Computing Service
+<br>
+Cambridge CB2 3QH, England.
+<br>
+</P>
+<br><b>
+REVISION
+</b><br>
+<P>
+Last updated: 06 March 2007
+<br>
+Copyright &copy; 1997-2007 University of Cambridge.
 <br>
-Copyright &copy; 1997-2005 University of Cambridge.
 <p>
 Return to the <a href="index.html">PCRE index page</a>.
 </p>