the {1} in this regex is redundant as far as I can tell. otherwise good solution!

lines like
.toLowerCase() // transform the word to lowercase to ignore case
might look good when you are learning, but a code should be readable by itself, the function toLowerCase is pretty descriptive by itself, so it shouldn't need comments.

I like that you didn't use .map(Number) in the version splits, because then you'd have two more unecessary loops.

I forked it for this exact same reason, could you comment on it?

I think this is the best one I've seen, I'd use Template literals in the return statement tho.