my $ip ="1.133.123.123";
if (IP_valid ($ip)) {print "$ip ok\n"} else {print "$ip bad\n"}

my $ip ="";
if (IP_valid ($ip)) {print "$ip ok\n"} else {print "$ip bad\n"}

my $ip ="1asdf1.133.123.123";
if (IP_valid ($ip)) {print "$ip ok\n"} else {print "$ip bad\n"}

my $ip ="12341.133.";
if (IP_valid ($ip)) {print "$ip ok\n"} else {print "$ip bad\n"}

my $ip ="not at all valid";
if (IP_valid ($ip)) {print "$ip ok\n"} else {print "$ip bad\n"}

sub IP_valid {
my $ip = shift;
    $ip =~/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/;
    foreach ($1,$2,$3,$4){
        if ($_  <256 && $_ >0) {next;}
        return 0;
    }
    return 1;
}