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.