javascript显示剩余字数

textbox:

<textarea id="feedback" class="int" onkeyup="displaySpareNumber(this,300)" onchange="displaySpareNumber(this,300)"  name="content">

显示剩余字数:

 <p id="spareNumberBox">还剩<span><input value="300" id="spareNumber" style="border:none; color:red; width:40px; text-align:center; background:none;" readonly="readonly"  /></span> 汉字</p>

javascript用于计算显示剩余字数(两个英文字母或数字算一个汉字):

function displaySpareNumber(_this,size)
{
    var spareNumber=document.getElementById("spareNumber");    
    var len=_this.value.replace(/[^\x00-\xff]/gi,'xx').length/2;
    var snum=Math.floor(parseInt(size)-len);        
    spareNumber.value=snum;
    if(snum<0)
    {
        if(_this.value.length!=len)
        {
            if((len-_this.value.length)>(size/2))
            {
                _this.value=_this.value.substring(0,size/2);
            }
            else
            {
                _this.value=_this.value.substring(0,size-(len-_this.value.length));
            }
        }
        else
        {
            _this.value=_this.value.substring(0,size);                
        }
        spareNumber.value=0;
        return;            
    }        
}