How To Add Currency In String?

Using Javascript 7 Regex Add Currency To This Given String......

function addCurrency( MyString )
{
  var objRegex = /-?[0-9]+\.[0-9]{2}$/;

    if( objRegex.test(MyString)) {
      objRegex.compile('^-');

      if (objRegex.test(MyString)){
        MyString = '(' + MyString.replace(objRegex,' ') + ')';
      }
       return '$' + MyString;
    }
    else
      return MyString;
}