title: Remove parenthesis from String using java regex
body: <p>I want to remove parenthesis using Java regular expression but I faced to error <code>No group 1</code> please see my code and help me.</p>
  
  <pre><code>public String find_parenthesis(String Expr){
          String s;
          String ss;
          Pattern p = Pattern.compile("\\(.+?\\)");
          Matcher m = p.matcher(Expr);
          if(m.find()){
              s = m.group(1);
              ss = "("+s+")";
              Expr = Expr.replaceAll(ss, s);
              return find_parenthesis(Expr);
          }
          else
              return Expr;
      }
  </code></pre>
  
  <p>and it is my main:</p>
  
  <pre><code>public static void main(String args[]){
      Calculator c1 = new Calculator();
      String s = "(4+5)+6";
      System.out.println(s);
      s = c1.find_parenthesis(s);
      System.out.println(s);
  }
  </code></pre>