Check that string is an IP address

addr_is_ip.go

// IsIp checks that the given string is IPv4 or IPv6 address.
func IsIp(addr string) bool {
	// IPv6
	if addr[0] == '[' {
		return true
	}
	// domain can have digits, but it can't have ONLY digits
	// So if all symbols are digits then this is an IPv4 address. We can check only first octet before dot .

	// end = min(addr len,3)
	end := len(addr)
	if end > 3 {
		end = 3
	}

	for k := 0; k < end ; k++ {
		char := addr[k]
		if char == '.' {
			break
		}
		// if not digit then this is not an IP
		if char < '0' || char > '9' {
			return false
		}
	}
	return true
}