Could someone tell me what this asterisk means?

Your solution has to compile the regex for every element in the array. You could compile it first to eliminate that overhead, but I suspect it would still be slower as it involves multiple calls to match instead of a single call. str.join and re.findall are both very efficient operations.

Python 3.8 adds the walrus operator which allows you to assign the result of the sum in the if statement to a variable and reuse it later in the comprehension.

The only difference is that in the linked kata is that the range has the following criteria:

It is not considered a range unless it spans at least 3 numbers.

and this can be achieved by adding just 1 line small line.

Actually, it's not. The two are not solvable with the same code.

Duplicate https://www.codewars.com/kata/51ba717bb08c1cd60f00002f/

You might find this article on the Python wiki useful:

https://wiki.python.org/moin/TimeComplexity

Take 2 strings s1 and s2 including only letters from a to z.

I read that as stating s1 and s2 will only include letters from a to z. ie. don't worry about non-alphabetic characters.