Create regex to check input field value and pass it if it is valid

So, are we supposed to check if it's valid or format it to become valid?

pass it if it is valid

What do you mean by "pass it"? Is it like return the input immediately or proceed with the extra formatting you didn't mention in the description?

Else it goes to error.

No it didn't. It expected a string instead. Either revise the description or fix the test.

No random tests ~~ so https://www.codewars.com/kata/reviews/566a93b343bc63e95f000098/groups/5fe2fe79cbd8150001746d17 passes easily 😒

Else it goes to error doesn't really mean much. Would you like an error to be thrown? To be returned? Or some specific value to be returned?

I'd recommend actually testing "9", as I don't think my solution actually handles that, though it passed.

Speaking of which, it would be a good idea to add randomly generated tests to the kata, to cover anything you may not have thought of.