Sometimes we need to replace whole word with the word boundaries in a string. For this we use "\b" regular expression token which is called a word boundary. It usually matches at the start or the end of a word.

Example: You just need to place the word between word boundaries

package com.demo;

public class TestApps {

    /*
      *@param args
     */
    public static void main(String[] args){

        String s = "one dog two catsdog dogCats in the field";

        //It will replace if the first character is a word character or not
        String d = s.replaceAll("\\bdog", "#@!*");
        System.out.println(d);

        //It will replace if  if the last character is a word character or not
        String b = s.replaceAll("dog\\b", "#@!*");
        System.out.println(b);

        //It will replace whole word only
        String c = s.replaceAll("\\bdog\\b", "#@!*");
        System.out.println(c);
        }
}

Output:

one #@!* two catsdog #@!*Cats in the field
one #@!* two cats#@!* dogCats in the field
one #@!* two catsdog dogCats in the field

Hope this will help:)