The interface is very clean, simple, and to the point. One suggestion though: It very quickly gives an "Invalid regular expression" error while I'm still typing. From the perspective of the regex parser that is certainly a correct assessment. However from a user's perspective, what's entered is simply incomplete. Although I like the instant evaluation/validation of the expression, it might be a good idea to increase the interval between when the last key is pressed and when the evaluation begins. The need to read the error message along with the textbox turning red while the regex is only incomplete causes an unneeded break in user interaction.

That aside, nice work! I'm sure I'll be using it. :)