验证TextBox里的内容必须是数字或空

function ValidateData()
{
  var e = window.event.srcElement;
  if(e.value!="")
  {
  if ( !((window.event.keyCode >= 48) && (window.event.keyCode <= 57)))
  {
  alert("卷数必须输入数字");
  e.value="";
  }
  }
  e.focus();
}

<asp:TextBox ID="TextBox4" runat="server" Width="152px" onkeypress="ValidateData();" MaxLength="9" >