Your explanation is extremely helpful, thank you and kudos for the clever solution.

This is a bit older now so you may not remember but was this your first thought off how you thought to solve this? Or was this a clever idea you thought of later?

It replaces any empty pairs of brackets until there are any of those left. If the initial expression is valid, all braces will be removed, therefore s will become an empty string and not s will evaluate to True; otherwise s will be non-enpmty and not s will evaluate to False.