I was working on a (FOSS) regex tester for desktop OSes using GTK and sourceview. Might pick the project back up soon, just gotta make sure that staying on GTK3 doesn't break my sourceview lib.

I prefer this one though no Ruby (but maybe Ruby is the same as one of their supported langs?)

https://regex101.com

Regext is one that I find quite helpful at times as well.

https://regexr.com/

+1 I use Rubular quite a bit myself.