When the question was "regex or regexp?"

For those wondering how: ? is a quantifier that matches preceding term (p in this case) exactly zero or one time. That means both "regex" and "regexp" can be matched by this filter.

By the way there's bunch of useful websites like https://regex101.com (my personal favorite) where you can check and debug regex interactively.
The best I can tell you about regex is: DO NOT WASTE YOUR TIME, it's not designed for mankind.

First serious program for a company I wrote was shitloaded with regex. Broke my nerves tbh