use strict;
use warnings;

while (<DATA>)
    {
    chomp;
    
    if (/^[a-zA-Z0-9]{6,20}$/) {
        print "Success: >$_<\n";
    } else {
        print "Invalid string: >$_<\n";
    }
}
    
__DATA__
Success
Invalid string!

##</code><code>##

Success: >Success<
Invalid string: >Invalid string!<