Validate if input string is a number between 0-255 using regex

You can use this regex:

boolean valid = IP.matches("\\b(1?[0-9]{1,2}|2[0-4][0-9]|25[0-5])\\b");

RegEx Demo

Tested this:

static String pattern = "^(([0-1]?[0-9]?[0-9]?|2[0-4][0-9]|25[0-5])\\.){3}([0-1]?[0-9]?[0-9]?|2[0-4][0-9]|25[0-5]){1}$";

It works for the following:

• IP Addresses xxx.xxx.xxx.xxx / xx.xx.xx.xx / x.x.x.x / mix of these.
• Leading zeros are allowed.
• Range 0-255 / maximum 3 digts.