Rubular: a Ruby regular expression editor (https://rubular.com/r/mP6IRzteSm) is another option which is pretty minimal but useful.
However it is closed source, and it sends your info to the server for processing. This is of course not (likely) nefarious, but it does give me a little pause, and means that you should never use it if the data you are putting into it is sensitive!. For example, don't test your API key parsing regex on it with real/active keys! Also for corporate dev you are (probably) violating company policy by using it.
Rubulex[1] is a neat open source clone that I use a lot. The only downside is I have to start it locally. One of these days I'll stand up a permanent instance, though I don't want to do that without auditing the code and I simply haven't had time to do that. If anyone has done so I'd love to hear about it. Scriptular[2] is an open source clone that uses javascript:
Side note: If anyone knows of or wants to build an Elixir regex tool in Phoenix/LiveView, I'd be willing to collaborate a bit and willing to host/maintain (and I'll pay for the VM/domain). I've already got some Phoenix apps running in prod so if you get it to work with `mix phx.server` I can take it from there (I know that operationalizing and devops isn't what usually interests most people). A non LiveView version would be cool too, but it seems like such a cool project to build with LiveView to be super responsive and show off what you can do (and also learn Elixir/Phoenix/LiveView with a simple app). Would be really neat to release an elixir-desktop[3] version too! My email is in my profile if you are interested.
[1]: https://github.com/ofeldt/rubulex