<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">還可以輸入15字數</span>

function setShowLength(obj, maxlength, id)
{
var rem = maxlength - obj.value.length;
var wid = id;
if (rem < 0){
rem = 0;
}
document.getElementById(wid).innerHTML = "還可以輸入" + rem + "字數";
}