Hi,
you can try this
<asp:TextBox ID="TextBox1" MaxLength="10" runat="server"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" ControlToValidate="TextBox1"
ValidationExpression="^(0|[1-9][0-9]*)$" runat="server" ErrorMessage="Please Enter Only Numerics"
ForeColor="Red"></asp:RegularExpressionValidator>