Unfortunately it seems to be broken now, but reAnimator was an awesome regex visualization tool (http://osteele.com/tools/reanimator/ http://osteele.com/tools/r

Unfortunately it seems to be broken now, but reAnimator was an awesome regex visualization tool (http://osteele.com/tools/reanimator/ http://osteele.com/tools/reanimator/). It let you enter a regex and would display the state machine similar to this, but then it would also let you enter a string and it would animate the progress of the string matching (or not) against the state machine as you enter each character.