Example for Allowing Only Numric in a textbox on Client side

<asp:TextBox ID="txt_tenderno"  onkeydown="return onKeyPressBlockNumbers(event)" onFocus="this.style.background ='yellow'" onBlur="this.style.background='white'" runat="server" ></asp:TextBox>

function onKeyPressBlockNumbers(e)
{
 var key = window.event ? e.keyCode : e.which;
 var keychar = String.fromCharCode(key);
 reg = /\d/;
 return !reg.test(keychar);
}

-----------------OR---------------------

<asp:TextBox ID="txt_tenderno"  onkeydown="return numeric(event)" onFocus="this.style.background ='yellow'" onBlur="this.style.background='white'" runat="server" ></asp:TextBox>
function numeric(e) {

var key = window.event ? e.keyCode : e.which;
   return ((key == 8) || (key > 47 && key < 58));
}