This is the regular expressions pattern I use to test whether string characters represent a month number (from 01 to 12):
(0[1-9]|1[0-2])
The expression part inside the brackets is broken into 2 possible parts divided by a vertical bar (or pipe symbol): 0[1-9] or 1[0-2].
If a string matches any of the 2 parts it is true.
The first one says that the 2-character string has to start with 0 and end with any number in the range from 1 to 9, e.g. 01, 02, 09
The second part matches any 2-character string that starts with 1 and ends with 0, 1 or 2, i.e. 10, 11, or 12
