<script type="text/javascript">
var ns6=document.getElementById&&!document.all

function restrictinput(maxlength,e,placeholder){
if (window.event&&event.srcElement.value.length>=maxlength)
return false
else if (e.target&&e.target==eval(placeholder)&&e.target.value.length>=maxlength){
var pressedkey=/[a-zA-Z0-9\.\,\/]/ //detect alphanumeric keys
if (pressedkey.test(String.fromCharCode(e.which)))
e.stopPropagation()
}
}

function countlimit(maxlength,e,placeholder){
var theform=eval(placeholder)
var lengthleft=maxlength-theform.value.length
var placeholderobj=document.all? document.all[placeholder] : document.getElementById(placeholder)
if (window.event||e.target&&e.target==eval(placeholder)){
if (lengthleft<0)
theform.value=theform.value.substring(0,maxlength)
placeholderobj.innerHTML=lengthleft
}
}


function displaylimit(thename, theid, thelimit){
var theform=theid!=""? document.getElementById(theid) : thename
var limit_text='<b><span id="'+theform.toString()+'">'+thelimit+'</span></b> Karakter Kalan'
if (document.all||ns6)
document.write(limit_text)
if (document.all){
eval(theform).onkeypress=function(){ return restrictinput(thelimit,event,theform)}
eval(theform).onkeyup=function(){ countlimit(thelimit,event,theform)}
}
else if (ns6){
document.body.addEventListener('keypress', function(event) { restrictinput(thelimit,event,theform) }, true); 
document.body.addEventListener('keyup', function(event) { countlimit(thelimit,event,theform) }, true); 
}
}

</script>

<table width="557" border="0" cellpadding="0" cellspacing="0">
<form name="sitekle"method="post">

          <tr>
            <td width="228">İnput : </td>
            <td width="329"><input type="text" name="input" id="input" size=20>&nbsp;&nbsp;
<script>displaylimit("document.sampleform.input","input",30)</script></td>
          </tr>    
          <tr><td></td><td><div align="right">
            <input type="submit" value="Gönder" disabled/>
            </div></td></tr>
      </form>
</table>

bu nu dene mühim olan kısım üstteki javascript kısmı onu
karaktersayici.js diye kutuphaneyi kayit edebilirsin kullanımı ise
<script>displaylimit("document.sampleform.input"," input",30)</script>
displaylimit fonk adı document.sampleform.input","input",30
buradan da
document bu sayfa sampleform da normalde form name sine verilmeli ama onsuz da yapabiliyo işini input ta
<input name=xxx buradaki isim input yine "input" adı 30 da kaçtane sayacaığını belirtiyor
https://www.r10.net/reputation.php?p=643971 rep vermene yardımcı olayım