js에서 이메일 형식이 올바른지 확인하는 방법

<form action="#"onsubmit="return validate_form(this);" method="post">    Email: <input type="text" name="email" size="30">
    <input type="submit" value="Submit">
</form>
<script type="text/javascript">    function validate_required(field,alerttxt)
    {        with (field)
        {            if (value==null||value=="")
            {alert(alerttxt);return false}            else {return true}
        }
    }    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_required(email,"请填入内容!")==false)
            {email.focus();return false}            if (validate_email(email,"请填写正确的邮箱格式!")==false)
            {email.focus();return false}
        }
    }
</script>