<script type="text/javascript" language="javascript">
        function onkey(textid)
        {
            var textBox = document.getElementById(textid.id);
            if(textBox.value.length>=5)//限定长度为5这个中文英文都适用
            {
                if(event.keyCode!=8&&event.keyCode!=16&&event.keyCode!=17&&event.keyCode!=37&&   event.keyCode!=38&&event.keyCode!=39&&event.keyCode!=40&&event.keyCode!=46)
                {
                    event.keyCode=9;
                    alert('文字太多了');
                }
            }
        }

    </script>