def is not the only way to define a function in Python.

how do i dont use 'def' and 'return' when codwars want to call my code with def?

I'm really surprised it was voted as 5kyu. I avoided it a few times thinking it would be much harder, but it really felt like 7kyu...

I was about to post this purplemath link. Mathematically challenged, I found it more helpful than the link in the instructions.

The short answer is, It's a Positive Lookahead (zero-width assertion). look ahead for 'space' (or end of $tring) without including it in the result. Good Job on this kata.