import java.util.regex.Pattern;

public class RegexReplaceString{
    public static void main(String[] args) {
        String str = "Hello, unko";

        // unko -> World
        System.out.println(Pattern.compile("(unko)$").matcher(str).replaceFirst("World"));

        // unko -> World
        System.out.println(str.replaceAll("(unko)$", "World"));
    }
}