Well done. I don't know what you think but I found that most of the time the character classes would intersect perfectly (i.e. there'd only be one character possible once you intersect both sides of a single square). That made it pretty easy overall since for the vast majority of the board you don't have to worry about the "context".

But I guess if it's meant for an audience of folks not very familiar with regexes it's difficult enough as it is.