regexr is pretty good as well. ;)
Thanks for posting this, it’s really nice.
It includes Python as a flavor, which many otherwise cool sites don’t.
I also really like the live-updated automatic explanation pane - very handy for checking if you’ve said what you meant.
Recently, I’ve started to use the Node REPL for quick tests of Regex due to JS' nice RegExp type and functions. (It helps when you’re actually learning JS too)