OP solved it, closing!

WTF? The solution I see just returns the 5 hardcoded sample tests. What did you expect?

If you have any actual code, you can post it here with spoiler flag, or somewhere on Discord CW server. But given that it is a puzzle kata, you're not likely to receive much help.

Read this: https://docs.codewars.com/training/troubleshooting#post-discourse

Duplicate issue (I'll try to fix this)

It was a Python2 solution.

In Python2, running filter() on a str returned a str directly.
Now, in Python3, as you said, filter() always returns an Iterator of type filter.

Regexp are parsed in the same way a text is parsed: from left to right.
The order of groups is determined by the position of the opening brackets.
When groups are nested, they are created from outside to the inside -
or from left to right if you keep only opening brackets

Try using regex101

Tests do not check for the structure of your solution. You can remove all code and just replace it with something what works, if you know how.

"find the smallest number you can get" 18786300289<18786302809

Hi EvgKrm! Look at the complete result of this test.

It can be computed, it's just really inefficient without memoization.