Came to say that, hyper clever really.

yes

because it doesn't "consume" what would otherwise be the second group. See:

    test.assert_equals(insert_dash(13579),'1-3-5-7-9')

Pretty interesting solution. I solved it without using REGEX

manual notification, see above

Great solution, could you please explain the pattern a bit?

From description:

Allowed characters are uppercase / lowercase latin letters and digits from 0 to 9

Returns True with non latin characters.

So to call the function, you'd have to do str.isalnum(foo)?

Wow

Maybe

Niiiiiice

damn! this is so pretty!