Let's check ip addresses in Java with regex

• First of all we need to import 2 library.

import java.util.regex.Matcher; import java.util.regex.Pattern; 

• Now, lets define ip regex

 public static final String IP_REGEX ="^" + "([01]?\\d\\d?|2[0-4]\\d|22[0-3])\\." + "([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\." + "([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\." + "([01]?\\d\\d?|2[0-4]\\d|25[0-4])$"; 

^ -->start of the line

( --> start of group #1

[01]?\d\d? --> Can be one or two digits. If three digits appear, it must start either 0 or 1
# e.g ([0-9], [0-9][0-9],[0-1][0-9][0-9])

| --> or

2[0-4]\d # start with 2, follow by 0-4 and end with any digit (2[0-4][0-9])

| --> or

25[0-5] # start with 2, follow by 5 and ends with 0-5 (25[0-5])

) --> end of group #2

. --> follow by a dot "."

.... --> repeat with 3 times (3x)

$ -->end of the line

• Create an pattern object and pass regex string to it.

 String ipregex = System.console().readLine(); //read input Pattern ipPattern= Pattern.compile(IP_REGEX); Matcher ipMatcher = ipPattern.matcher(ipregex); 

• Finally we can control our result.

 //Control flow if (!ipMatcher.matches()) { System.out.println("Yay ! "); } else{ System.out.println("Noooooo! "); } return ipregex;