dom选择器自己写

function $$(tag,className){
var eles = document.getElementsByTagName(tag),i,len,_arr=[],reg=new RegExp("(^|\\s)"+className+"($|\\s)");
for(i=0,len=eles.length;i<len;i++){
if(reg.test(eles[i].className)){
_arr.push(eles[i]);
}
}
eles = null;
return _arr;
}