This was a fun one. I tried my best on this one to create a solution that doesn't use Array functions like map, reduce, split and join since I've been relying on those quite a bit in many of my solutions to other katas. I was able to get this one worked out by relying almost exclusively on the power of regex and less on Arrays.

Hmm, looking through some of these solutions I think most of them would do the same if passed an empty string or a string made up completely of invalid characters.