I got a bug report from the #haskell IRC channel last night that empty ByteStrings were broken. The new bugfixed versions of the affected packages are: regex-posix-0.92 regex-posix-0.72 regex-pcre-0.92 regex-pcre-0.81 All the above have been uploaded to http://hackage.haskell.org/ and are available via darcs (see below). Details: regex-posix-0.92 replaces the "unstable" regex-posix-0.91 that I announced yesterday. The bug fix also applied to ByteString.Lazy repository: http://darcs.haskell.org/packages/regex-unstable/regex-posix/ regex-posix-0.72 replaces the regex-posix-0.71 that came with GHC 6.6 repository: http://darcs.haskell.org/packages/regex-posix/ The repository version is setup to compile alongside GHC in some way that I cannot easily package, so the tarball on hackage has edited Wrap.hsc and cabal files. regex-pcre-0.92 replaces the "unstable" regex-pcre-0.91 that I announced yesterday. The bug fix also applied to ByteString.Lazy repository: http://darcs.haskell.org/packages/regex-unstable/regex-pcre/ regex-pcre-0.81 replaces the "stable" regex-pcre-0.80 that is compatible with the regex-base-0.71 supplied with GHC or regex-base-0.72 that I announced yesterday. repository: http://darcs.haskell.org/packages/regex-pcre/