<script type="text/javascript">
function validate(key) {
//getting key code of pressed key
var keycode = (key.which) ? key.which : key.keyCode;
var phn = document.getElementById('txtPhn');
//comparing pressed keycodes
if ((keycode >= 65 && keycode <= 90) || (keycode >= 97 && keycode <= 122) || (keycode >= 48 && keycode <= 57) || keycode == 44 || keycode == 46 || keycode == 47)
return true;
else
return false;
}
</script>
<asp:TextBox ID="Textbox1" onkeypress="return validate(event)" runat="server"></asp:TextBox>