Working through samples from the Great Computer Language Shootout, I wrote a Haskell program which is supposed to be basically equivalent to the perl program at: http://www.bagley.org/~doug/shootout/bench/regexmatch/regexmatch.perl And in fact I copied the regular expression directly from the perl example and used search and replace to change single backslashes to double backslashes to deal with Haskell vs. Perl strings. But the regex fails to match where it should. Since I copied the regex directly from the working perl script, I'm pretty sure it's correct. Since the RegexString modules says the regex syntax is that of Perl, I'm reporting it as a bug. The haskell file is attached. It is intended to be run with an integer command line argument (2 is fine), and input piped from the file at: http://www.bagley.org/data/shootout/regexmatch/Input I'm on Windows NT 4.0 sp4 and GHC 5.02.