After importing Text.Regex and Data.Maybe, here's some output from ghci:

*ProtoRegexp> fromJust $ matchRegex (mkRegex "foo") "barfoobaz" [] *ProtoRegexp> fromJust $ matchRegexAll (mkRegex "foo") "barfoobaz" ("ba","foo","baz",[])

I'm relatively sure those are both wrong. I think the first should be "foo" and the second should be ("bar","foo","baz",[])

I haven't yet tried Text.Regex.Posix to see if the problem is just the wrapper.

The first example is correct (it returns the parenthesised sub-expressions in the match). The second is indeed incorrect, there's an off-by-one in calculating the first component of the tuple, which I've now fixed. Thanks for the report. Cheers, Simon