Although it's overkill when I could have a reference sheet on my computer or on the wall, but I always remember that regexr has its cheat sheet on the side that I can easily get to. I use their cheat sheet more than the actual editor on their page.

It looks like regex101 has it as well, but the regexr one is straight to the point (at least for me). https://i.imgur.com/Hg9jGOa.png