It look like rubular just shows you example matches out of text you've already typed in (the same situation you have when playing with any regex matching library directly), whereas regexio generates matches you might not expect out of the blue. Can you clarify what sense of similarity you mean to point out?

Yes, I realize that rubular doesn't generate the matches. But they are both web sites where you primarily enter a regex in order to explore its behavior. The similarity is obvious.

And the Python 're' option: http://pythex.org/