Actually, that wouldn't solve the problem.
Using tr was a good suggestion when Slug wanted to count the characters, but in this case he just wants to eliminate them.
From the documentation

If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class.

So nothing would be replaced.

You are right, of course, on the '.=' operator.