In ghci

>guessKeys "modF1"
"*** Exception: Test.hs:8:10-85: Irrefutable pattern failed for pattern (_,
_, _
, [key])

Because, the pattern "xK_(\\w+)" does not match with your input "modF1", as you already tried in the bbb function.

*Main> bbb "modF1"
("modF1","","",[])

Note that the 4th component is an empty list, which does not match with your specify in
2nd where clause
(_, _, _, [key]) = line =~ "xK_(\\w+)" :: (String, String, String, [String])

> aaa "modF1"
<interactive>:1:0:
No instance for (Text.Regex.Base.RegexLike.RegexContext
Regex [Char] [[a]])
arising from a use of `aaa' at <interactive>:1:0-10

You need to give a type annotation to the function aaa so that the type class instance can be "resolved".

Regards,
Kenny