Remove parenthesis from String using java regex

I want to remove parenthesis using Java regular expression but I faced to error No group 1 please see my code and help me.

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;
    }

and it is my main:

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);
}