Yeah, I agree. Whenever I use them, I always comment on what it means. It's a very clever solution and I couldn't have come up with it (I used recursion) but practically unmaintainable.

Could you elaborate on why this is not efficient code ? I do think it is a bit clever as the map function arguments are not explicit enough and you would have to have a deeper understanding of the arguments it takes to know what it all means but it does seem like an efficient solution.

Clever but it would drive me mad in production code. RegExp is one of the most complicated things to understand. Even if you are well versed in it you still have to decypher the the regex string.