<script type="text/javascript">
//123213@qq.com
//dancheng@163.com
//_tom123.77@hahaha.com
var reg = /^\w[a-zA-Z0-9_\.]{4,9}@\w+(\.[a-z]+)+$/;
var str = "98239223@qq.com"; //结果:["98239223@qq.com", ".com", index: 0, input: "98239223@qq.com"]
var str = "sdasdasd"; //结果:null
var str = "_tom123.77@hotmail.com"; //结果:["_tom123.77@hotmail.com", ".com", index: 0, input: "_tom123.77@hotmail.com"]
var res = str.match(reg);
console.log(res);
</script>