Editing Regular expressions (section)

== Sample benchmark ==
The benchmark consists of matching "Hello world foo=123 whereas bar=456 Goodbye" against ".*foo=([0-9]+).*bar=([0-9]+).*" (as a lazy bytestring) 1mln times (along the lines of length . filter matchesPat . replicate 1000000).
It has been performed on a 2x2GHz P4 machine, compiled with -O2, ghc 6.10.1.

regex-pcre: 0.02s

pcre-light: 0.02s

regex-tdfa 1.0.0 (no compiler bug): 0.03s

regex-dfa 0.91: 5.4s

regex-posix: 20s

regex-tdfa 1.0.0 (most probably compiler bug): 89s

In all cases, the pattern is compiled only once.

WARNING: This result of 97s may be the result of a compiler bug that probably may cause the pattern to be compiled at every match invocation; without that bug, regex-tdfa performs extremely well. Please see ticket http://hackage.haskell.org/trac/ghc/ticket/3059 

UNWARNING: The code for regex-tdfa 1.0.0 is being improved. In particular the slow 89s is probably real.  But it need not be so.  The upcoming version runs in less than 0.2 seconds.

Thus, for patterns as simple as this one, it is appropriate to use pcre.
For patterns where the Perl-style strategy of backtracking causes exponential backtracking blowup, it is appropriate to use regex-dfa or regex-tdfa.