Best solution! I hope one day I will understand regexp so that I can write equally elegant solutions.

should check for non string argument

I didn't know you could do that lol

:) same here

It really depends on the engine, whether it's compiled (and also the compiler in that case).

Not only is it more readable, it increases efficiency. The other regex (for an arbitrary string of 16 characters) ran 114 steps, this one ran 72. Nice work.

It might depend on the regular expression engine, but I'd imagine a fixed-width look-around can be optimized to some extent; and I think I've run into it where engines don't support variable-width look-around. Only engine I've really looked at code for is Python's and all the goto statements in there make it pretty hard to follow. What I learned there is that you can avoid regular expression DOS with system interrupts using Python's regular expression engine.

Bruh , I can't understand anything.

i wish i could write code this elegant

webmisfit, I wouldn't put him down for his reasoning as it makes sense in a real-world context; this kata definitely could be built upon to incorporate data normalization, and such a kata, say a part 2, could also potentially make a good exercise for the practice of separation of concerns.