How to represent regex number ranges (e.g. 1 to 12)?

I tend to go with forms like [2-9]|1[0-2]? which avoids backtracking, though it makes little difference here. I've been conditioned by XML Schema to avoid such "ambiguities", even though regex can handle them fine.

Yes, the correct one:

[1-9]|1[0-2]

Otherwise you don't get the 10.