Button submit

I have a validation for my email address that is working. However, if I type nothing in and click submit,that works. How would I disable the button until something is typed in?
input type="text" class="textbox" runat="server" id="txtEmail" /><asp:Button Text="" ID="btnSave" runat="server" class="btn" /> <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtEmail" fontcolor="red" ErrorMessage="Please enter a valid e-mail" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ForeColor="white"> </asp:RegularExpressionValidator&g ...