Implement custom regexp for label matching

I realized I don't really **want** to use this, so it took a while. Who knows, maybe now that it's here, I will use it. @cobyism if you're still interested in this function, it'd be awesome if you can try out this branch some time! :heart:

Fixes #2.

![image](https://f.cloud.github.com/assets/1153134/2057842/83fd3bf4-8b51-11e3-84e4-f55ec31e665c.png)