1. The delay between updating the regex/test text and seeing the result is a little long. I find regex tools much more useful if I can see the results change as I'm typing.
2. I did a simple regex with two capture groups and the second backreference came up red in the matches box. As soon as I see red, I assume there's an error. Might be worth removing those "danger colours" from the list.
Very clean design and targets the niche nicely. One of my pet peeves is broken wordcounts though. A single space registers on your site as 1 word. Can I suggest checking out my post about javascript wordcounts? http://drewschrauf.com/blog/2012/06/13/javascript-wordcount-...
I tried setting up ruby/octopress on my mac last night and it was a pain in the ass. Great to see a node.js alternative. With a plugin infrastructure it'll be a serious contender to Jekyll.