The version 0.71 of regex-base supplied with GHC 6.6 has a bug in Text.Regex.Base.Context:

instance (RegexLike a b) => RegexContext a b (MatchResult b) where

that essentially uses 'tail' where it should not, and this may even be using tail on an empty list. This has been patched in the just announced unstable version of regex-base 0.83 and this message announces that the stable repository at http://darcs.haskell.org/packages/regex-base/ now has version 0.72 which is just 0.71 with this bug fixed. For the eventual GHC 6.6.1 release, the supplied version of regex-base should either be conservatively updated to 0.72 or updated to the slightly more convenient regex-unstable/regex-base version (currently 0.83). Cheer, Chris Kuklewicz