WPF implementation of a Regular Expression builder/designer

When WPF first came out I wanted to find something to "force" me to learn WPF. So I picked something that was well beyond "Hell, World!" but still not terribly complicated. I came across software called Rad Regex Designer (originally here but apparently not around any more) and I thought it worked pretty well, so I decided to use it as "inspiration" as I created something "fairly similar" in WPF.

These days when I'm cobbling something together that's regex-dependent, it's generally in Powershell so I'll just hack on it there. But this tool works pretty well, so I do use it occasionally. But there are a couple of weird things, esp. with the application's interface.