This simple JavaScript function validates the TextBox input that allows only numeric values to be entered.
<script type ="text/javascript" language="javascript" >
function ValidateText(i)
{
if(i.value.length>0)
{
i.value = i.value.replace(/[^\d]+/g, '');
}
}
</script>
In your textbox add the onkeyup event or onkeypress.
<asp:TextBox ID="TextBox1" runat="server" OnTextChanged="TextBox1_TextChanged" onkeyup = "javascript:ValidateText(this)"></asp:TextBox>
Note: If you wan't to validate a TextBox that allows negative/positive numeric values with two decimal places then you can refer to this links below:
http://forums.asp.net/p/1323787/2645052.aspx#2645052
http://forums.asp.net/t/1356834.aspx