thanks :)

From Data.List documentation:

In the result of xs \\ ys, the first occurrence of each element of ys in turn (if any) has been removed from xs.

In my solution, the first parameter to \\ is the alphabet set, thus punctuations and numbers are ignored, as they don't exist within the alphabet set.

Nice solution, but I don't understand how this works with non-letter characters (like punctuation and numbers) ?