请教:对象还是正则表达式的问题?

[KKK111000]

<script type="text/javascript">
<!--

var obj =
{
a:"a1",
b:"b1",
c:"c1"
}

var s = "ka"
//alert(s.replace(/k([abc])/,'obj["$1"]'));
alert(s.replace(/k([abc])/,obj["$1"]));

//-->
</script>

为什么结果为undefined?

[hbhbhbhbhb1021]

<script type="text/javascript">
<!--

var obj =
{
a:"a1",
b:"b1",
c:"c1"
}

var s = "ka"
//alert(s.replace(/k([abc])/,'obj["$1"]'));
alert(s.replace(/k([abc])/,function(a,b){return obj[b]}));

//-->
</script>

[qf521lx]

alert(s.replace(/k[abc]/, obj['a']));