Java Regex to Validate IPv4 Address

In this article, we’ll learn how to validate IPv4 addresses using Java Regex

IP Address Format

Let’s first take a look at typical IPv4 address examples:-

0.0.0.0
172.16.254.1
255.255.255.255
192.168.0.1
192.168.1.255

We have the following observations from the above examples:-

1. A valid IPv4 Address is in the form of A.B.C.D
2. The length of A, B, C, and D lies between 1 to 3 digits
3. The value of A, B, C, and D lies between 0 to 255
4. Leading 0’s not allowed

Regex to validate IP Address

^((\\d|[1-9]\\d|1\\d\\d|2[0-4]\\d|25[0-5])\\.?\\b){4}$

Where,

1. \\d to match single-digit numbers between 0 to 9
2. [1-9]\\d to match numbers between 10 to 99
3. 1\\d\\d to match numbers between 100 to 199
4. 2[0-4]\\d to match numbers between 200 to 249
5. 25[0-5] to match numbers between 250 to 255
6. (\\d|[1-9]\\d|1\\d\\d|2[0-4]\\d|25[0-5]) to match numbers between 0 to 255
7. \\.? to match the dot operator . after each number between 0 to 255
8. \\b is a word boundary
9. {4} to match 4 sets of such numbers (i.e. between 0 to 255)

We can further concise the above regex taking the common \\d out:-

^(((|[1-9]|1\\d|2[0-4])\\d|25[0-5])\\.?\\b){4}$

Validate IP address in Java

Let’s look at the Java method to validate the IPv4 addresses using the above regex:-

public static boolean validateIPv4Address(String ipv4){
  String regex = "^(((|[1-9]|1\\d|2[0-4])\\d|25[0-5])\\.?\\b){4}$";
  Pattern pattern = Pattern.compile(regex);
  Matcher matcher = pattern.matcher(ipv4);
  return matcher.matches();
}

Let’s use the above method to run test cases:-

1. Test valid IPv4 addresses:-

   @Test
   public void validateIPv4Address_validIPv4Addresses(){
     assertTrue(validateIPv4Address("0.0.0.0"));  // pass
     assertTrue(validateIPv4Address("127.0.0.1")); // pass
     assertTrue(validateIPv4Address("192.168.10.1")); // pass
     assertTrue(validateIPv4Address("172.16.254.1")); // pass
     assertTrue(validateIPv4Address("192.168.1.255")); // pass
     assertTrue(validateIPv4Address("255.255.255.255")); // pass
     assertTrue(validateIPv4Address("10.98.30.56"));  // pass
   }
   

2. Test invalid IPv4 addresses:-

   @Test
   public void validateIPv4Address_invalidIPv4Addresses(){
     assertFalse(validateIPv4Address("192.168.0.256")); // value above 255
     assertFalse(validateIPv4Address("192.168.0.01")); // leading 0 in 01
     assertFalse(validateIPv4Address("192.168.0")); // only 3 sets, 4th missing
     assertFalse(validateIPv4Address(".192.168.0.1")); // starts with .
     assertFalse(validateIPv4Address(".192.168.0.1.")); // ends with .
   }