If we describe these here too carefully, the computer will begin to seem to understand them as well as we do.

See Foldoc:regular+expression.

Months: there are twelve of them, the regular expression for months is being worked on by folks reading this wiki.

Current incarnation:

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

Um, what?

That's either 0 followed by any number between 1 and 9; or 1 followed by a number between 0 and 2.

No, why are you creating a regex for months? Or, more particularly, what exactly do you want to represent? Regexes are easy.