Piggybacking onto this to mention my go-to online RegEx editor: RegExr. It lets you test the regex as you type, explains the particular symbols used, as well as has a sidebar where you can see different pattern types categorically. I’ve been using it for almost 2 years now, and haven’t had any reason to use much else (after I discovered this).
Not sure if this is the one you’re referring to, but NileRed made a video on this:
https://youtu.be/h8OB-GKWjrc