The value.match documentation on the Open Refine github wiki is a bit daunting, so I'm going to try to break it down in this post.
Some programming speak first: GREL functions usually yield some sort of data after they're done doing their thing. That data is called a return value. So, that's what I'm referring to whenever I make statements like, "match returns <whatever>".
Anyway, the biggest hurdle to my understanding was figuring out how match and regular expressions worked together. Here's what I've determined:
Unlike Marcedit, you have to account for the whole line if you're using regex in match. 