Rubular is awesome for working with regexes and testing things out. It's fast, simple, and allows you to write/test/debug regexes super quick. The bummer is that it's not open source, so taking a peek under the hood to learn how its built isn't possible. (insert sad face)
Here are two options on GitHub that claim to be copycats. I have yet to dig deeper, but I plan to.
If you're the owner of Rubular, we'd love it if you would open source the code.