function heightlight(string, keyword) {

//如何实现???  

var reg = new RegExp(keyword, "gi");

//string = string.replace(reg, "<span class='heightlight'>$1</span>");

string = string.replace(reg, "<span class='heightlight'>"+keyword+"</span>");

return string;

}
//比如
var str="aaABbac";
var result=heightlight(str,"AabB");
//如何才能实现返回以下的结果:

//"a<span class='heightlight'>aABb</span>ac"

//如果string = string.replace(reg, "<span class='heightlight'>$1</span>")能实现那就方便多了，但replace()方法的字符$1这样用好像不行，求教各位大神指点！！！！！！