js验证email的正则

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> 
<html> 
    <head> 
        <meta http-equiv="Content-Type" content="text/html; charset=GB2312" /> 
        <title>Untitled Document</title> 
        <script language="JavaScript" type="text/javascript"> 
            function $(obj){ 
                return document.getElementById(obj); 
            } 
            function checkEmail() 
            { 
                var str = $("myText").value; 
            //如果为空，则通过校验 
                if(str == ""){ 
                     alert("输入为空"); 
                    return; 
                 } 
                 
             if (str.charAt(0) == "." || str.charAt(0) == "@" || str.indexOf('@', 0) == -1|| str.indexOf('.', 0) == -1 || str.lastIndexOf("@") == str.length-1 || str.lastIndexOf(".") == str.length-1) 
                  alert("不是一个邮箱"); 
                 else 
                 alert("是一个邮箱"); 
            } 
        </script> 
    </head> 
    <body> 
        <input type="text" id="myText" /> 
        <input type="button" onclick="checkEmail()" value="验证是否邮箱" /> 
    </body> 
</html>

[Ctrl+A 全选注:如需引入外部Js需刷新才能执行]