how to use regex in replaceall in java code example

Example 1: java string regexp replace

String input = "abcd";
Pattern p = Pattern.compile("regexp");
String output = p.matcher(input).replaceAll("replacement");

Example 2: replaceall in java

public class ReplaceAllExample{  
public static void main(String args[]){  
String s1="Google is a very good website";  
String replaceString=s1.replaceAll("a","e");//replaces all occurrences of "a" to "e"  
System.out.println(replaceString);  
}}