regex - 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 strict;
use warnings;

my $pattern;
my $month = "(0[1-9]|1[012])";
my $day = "(0[1-9]|[12]d|3[01])";

system("cls");

do {

    print "Enter in a month: ";
    chomp($pattern = <STDIN>);

    # We only want to print if the pattern matches
    print "Pattern matches
" if ($pattern =~ /$month/);


} while ($pattern ne "Q");

When I run this, it correctly filters from 01-12 but when I change the regex to:

$month = "(0?[1-9]|1[012])";

then the regex allows 13, 14, etc... what gives?

Answer 1

If you really like to use regex, you need to put ^ and $, like

"^(0?[1-9]|1[012])$"

it will not match 13, 14 ....