You're only asked to return True if n is a digit, which is what the solution does. Nowhere in the kata description does it say you have to use regex.

I would argue in any case that it's better practice to use built-ins for this instead of bringing in the overhead of the regex library where it's not required.

submit your answer adding:
print("\n".join(map(str,n))

this shows which tests are passing or not

I tried with regex first and it failed one miserable test. I have no way to check which one, so I decided to take a shortcut. (=:

That's the beauty of it, as regex is not enforced ;-)

1: SPOILER FLAG! / 2: that's not ternary stuff, that's normal boolean expression.