sub IdentityCardOkay
{
    my $num = shift;

    my($auth, $idx, $csum1, $lang, $bdate, $csum2, $cdate, $csum3, $csum4) = $num =~ /(\d{4})(\d{5})(\d)(\w)<+(\d{6})(\d)<+(\d{6})(\d)<+(\d)/;

    croak 'No german identity card!' unless lc($lang) eq "d";

    return (CheckSum("$auth$idx") == $csum1 and
            CheckSum($bdate) == $csum2 and
            CheckSum($cdate) == $csum3 and
            CheckSum("$auth$idx$csum1$bdate$csum2$cdate$csum3") == $csum4);
}

sub CheckSum
{
    my $num = shift;
    my $len = length($num);
    my ($n7, $n3, $n1) = (0, 0, 0);

    for(my $i = 0; $i < $len; $i += 3)
    {
        $n7 += substr($num, $i, 1);
        $n3 += substr($num, $i+1, 1);
        $n1 += substr($num, $i+2, 1);
    }

    return ($n7*7 + $n3*3 + $n1) % 10;
}