js邮箱验证

<html>
<head>
<script type="text/javascript">
function validate_email(field,alerttxt)
{    with (field)
    {    apos=value.indexOf("@")
       dotpos=value.lastIndexOf(".")
      if (apos<1||dotpos-apos<2)
      {alert(alerttxt);return false}
      else {return true}
     }
}
function validate_form(thisform)
{    with (thisform)
{    if (validate_email(email,"邮箱格式不下确!")==false)
{email.focus();return false}    }    }
</script>
</head>
<body>
<form action=""    onsubmit="return validate_form(this);"    method="post">
Email: <input type="text" name="email" size="30">
<input type="submit" value="Submit">
</form>
</body>
</html>