It's not 100 % clear what exceptions you expect, but I presume you are trying to handle the case when the regex does not match. I suggest handling it this way:

      #Transformation #1
      match = re.match(pattern, inword)
      result = match.group() if match else None

A possible explanation to your program suddenly slowing down is that regexes can suffer from catastrophic backtracking.

It's not 100 % clear what exceptions you expect, but I presume you are trying to handle the case when the regex does not match. I suggest handling it this way:

      #Transformation #1
      match = re.match(pattern, inword)
      result = match.group() if match else None

"after processing a few files, the script slows down" Have you considered the possibility that a particular file, or even a particular line is slow to process? A possible explanation to that would be that regexes can suffer from catastrophic backtracking.