How can I use a regular expression to validate month input?

Question

I am setting up this example Perl snippet to validate for months in a date:

Some scenarios I want to accept are:

MM M

#!/usr/bin/perl
use str         


        

Answer 1

"^(1[012]|0?[1-9])$" would be better because regular expression is assessed first one first. Let's say you want to match '12' and you write "^(0?[1-9]|1[012])$", then '1' will be picked because 0?[1-9] is taken first.