In addition to the online tools that other commenters have pointed out, I strongly recommend finding a regex "cheat sheet" (there are dozens online) and actually printing it out - I have one pinned to the corkboard next to my monitor and it is invaluable. It will save a lot of time from having to remember all the character classes, which characters do/don't need an escape \, and crazy lookaround syntax ("was it ?<! or ?<=")
I'll add Debuggex to the list that does a visual representation of a Regex and can be a god-send. https://www.debuggex.com