Fairly new to regex myself. I wondered the same after reading your comment and found this SE thread very informative.
http://programmers.stackexchange.com/questions/113237/when-you-should-not-use-regular-expressions

Never mind - set comprehension! Duh!

how does this pass the test case where there is a duplicate in the input list? I don't see how this comprehension eliminates repeating elements...

eg:
a1 = ['a','a',b']
a2 = ['ha', 'ba']
why is the output NOT ['a','a','b']?

I see that almost all the solutions are Regex based. I have heard that regex should be avoided unless there is a very strong reason to use it. Here, the regex is simple and quite easy to get right but I'm wondering why we WOULD or WOULD_NOT choose a regex in the wild

The input may have 0's in unexpected places - try making the ranges 0-9 and see if that is the issue...

Good point - I had not conisdered the "N times" instruction. That would be very hard to accomplish with a zero lenght string!

Thank you, this function really doesn't work in this case. However, there is the following statement
in the task: "reverse function should be executed only N times", so this case is impossibe, but thank you again, I changed my solution.

I see this failing for a zero length string so this not be one of the test cases. Fix it simply...

return str if len(str) <= 1 else str[-1]+reverse(str[:-1])