use strict;
use warnings;

my @strs = ("abcdefghijklm\n\n", "abcdefghijklm\n");

for (@strs)
    {
    if (/^[a-zA-Z0-9]{6,20}$/) {
        print "Success: >$_<\n";
    } else {
        print "Invalid string: >$_<\n";
    }
}