How do make the texbox accept only numerics, del key and backspace?

I always prefer to do it via client side Javascript... It is faster than using code behind

<asp:TextBox ID="txtNumber" runat="server" onkeypress="AllowNumeric();" MaxLength="8"></asp:TextBox>

Choose the Javascript you prefer.

function isNumber(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}
function AllowNumeric() {
  if ((event.keyCode > 47 && event.keyCode < 58) || (event.keyCode == 46) || (event.keyCode == 45)) {
    event.returnValue = true;
  }
  else {
    event.returnValue = false;
  }
}