textarea提示还能输入多少字

<textarea name="content" id="content" style="width:345px;height:120px;"></textarea><div id="contentwordage"></div>
<script type="text/javascript">
$(document).ready(function(){
    var limitNum = 500;
    var pattern = ‘还可以输入‘ + limitNum + ‘字符‘;
    $(‘#contentwordage‘).html(pattern);
    $(‘#content‘).keyup(
    function(){
        var remain = $(this).val().length;
        if(remain > 500){
                pattern = "字数超过限制！";
            }else{
                var result = limitNum - remain;
                pattern = ‘还可以输入‘ + result + ‘字符‘;
            }
            $(‘#contentwordage‘).html(pattern);
        }
    );
});
</script>