动态显示可输入的字数提示还可以输入的字数

<input name="title" type="text" size="50" value="{$p.title}" maxlength="15" onkeyup="javascript:setShowLength(this, 15, 'cost_tpl_title_length');"><span class="red" id="cost_tpl_title_length">还可以输入15字数</span>

function setShowLength(obj, maxlength, id) 
{ 
var rem = maxlength - obj.value.length; 
var wid = id; 
if (rem < 0){ 
rem = 0; 
} 
document.getElementById(wid).innerHTML = "还可以输入" + rem + "字数"; 
}