一个JS正则的字符串替换函数

function replacestring(oldstr,newstr,text)
{
 
      var exp = new RegExp(oldstr,'g');
      var c=text.replace(exp,newstr);
      return c;
}
 var oldstr='world';
var newstr='cccc';
var text='Hello world, Hello world';
var aa= replacestring(oldstr,newstr,text);
 console.log(aa);