title: how to replace some character's of String
body: <p>I have a numeric String i want to change the String first three character's by another character</p>
  
  <p>Suppose I have a String like  <code>0333XXXXXXX</code> and <code>+38333XXXXXXX</code>. if the first character of String is <code>+38</code> i want to replace it with "0" while remaining String remain the same Here is my code</p>
  
  <pre><code>        private String modifyNumber(String num) {
                  if (num.startsWith("+38")) {
                      num.replaceFirst("+38", "0");}
                     return num;}
  </code></pre>
  
  <p>"+38" is not accepting in <code>num.replaceFirst</code>.</p>