判断【textarea 】输入的字符

我想只要输入70个字符多了就不行！ JS判断吗？上代码。不会JS
<textarea name="Msg" id="Msg" runat="server" wrap=physical class="formstyle" cols="55" rows="6" >http://www.cqmjzx.com/</textarea> --------------------编程问答-------------------- 去网上下载个jquery 插件很好很强大，我有，但是没在公司！去百度一下把，应该很多的！ --------------------编程问答--------------------

<script language="javascript">

             var otext = document.getElementById("content");

             var otextleft = document.getElementById("textleft");

             var re = /[^\x00-\xff]/g;

             var maxLength = 1000;

             window.onload = function(){

                  if(document.all)

                         otext.onpropertychange = textleft;

                  else

                         otext.onkeypress = textleft;

                 }

             function textleft()

             {

                         var str = otext.value;

                          tr = str.replace(re,"xx");



                         otextleft.innerHTML = maxLength - str.length;

              }

       </script>

--------------------编程问答-------------------- Try:

<textarea   rows= "4 "   cols= "30 "   id=test
  onkeypress=doKeypress()
  onkeydown=doKeydown()
  onbeforepaste=doBeforePaste()
  onpaste=doPaste()
> </textarea>

<script   language=javascript>
maxLength   =   20
element=document.all[ "test "]

function   doKeypress()
{
  if(!isNaN(maxLength))
  {
    maxLength   =   parseInt(maxLength)
    var   oTR   =   element.document.selection.createRange()
    if(oTR.text.length   > =   1)
      event.returnValue   =   true
    else   if(element.value.length   >   maxLength-1)
      event.returnValue   =   false
  }
}

function   doKeydown()
{
  setTimeout(function()
  {
    maxLength   =   parseInt(maxLength)
    if(!isNaN(maxLength))
    {
      if(element.value.length   >   maxLength-1)
      {
        var   oTR   =   window.document.selection.createRange()
        oTR.moveStart( "character ",   -1*(element.value.length-maxLength))
        oTR.text   =   " "
      }
    }
  },1)
}

function   doBeforePaste()
{
  if(!isNaN(maxLength))
    event.returnValue   =   false
}

function   doPaste()
{
  if(!isNaN(maxLength))
  {
    event.returnValue   =   false
    maxLength   =   parseInt(maxLength)
    var   oTR   =   element.document.selection.createRange()
    var   iInsertLength   =   maxLength   -   element.value.length   +   oTR.text.length
    var   sData   =   window.clipboardData.getData( "Text ").substr(0,   iInsertLength)
    oTR.text   =   sData;
  }
}
</script>

补充：.NET技术 ,  ASP.NET