Which is interesting, as a proper regular expression is incapable of testing for a parens match. The set of strings with properly matched parens is a context-free language, not a regular language. But modern "regex" engines are capable of matching non-regular languages, and iterative regex-based substitutions (as his client-side code does--his server-side code is equivalent to the stack-based approach) can approximate the effect.

while he's using regex, it's hardly a regex-based solution. He's just striping out characters and parenthesis pairs