restrict textbox to allow only numerics

function numerics()
{
 key = String.fromCharCode(window.event.keyCode);
 if (!(key >= '0' && key <= '9'))   window.event.keyCode=0;
}
<asp:TextBox ID="txt1" Text=""  runat="server"  onkeypress=" return numerics();"></asp:TextBox>

function isAlpha(keyCode)
{
    return ((keyCode >= 65 && keyCode <= 90) || keyCode == 8||keyCode==37||keyCode==39||keyCode==9||keyCode==46)
}

function isNum(keyCode)
{
    return((keyCode < 105 && keyCode > 96)||(keyCode < 57 && keyCode > 48) || keyCode == 8||keyCode==37||keyCode==39||keyCode==9||keyCode==46)
}

<asp:TextBox ID="txtUserName" runat="server" onkeydown = "return isAlpha(event.keyCode);" onpaste =  "return false;" Width="130px"></asp:TextBox>

public static bool IsNumeric(String strVal)
    {
        Regex reg = new Regex("[^0-9-]");
        Regex reg2 = new Regex("^-[0-9]+$|^[0-9]+$");
        return (!reg.IsMatch(strVal) && reg2.IsMatch(strVal));
    }