I think a textbox and button with the label "Try it out" would be a nice feature for the dialog form when you add regular expressions.

Putting in a value and clicking the "Try it" button would validate the text with the rules presented and say "matched" "doesn't match" or something.

This would help the user know that it was working as planned.

Thanks,

Shane Story

Sorry, I never realized the the button with the "..." offered this