RequiredValidator ?

[Rio]

<asp:TextBox id="TextBox_search" runat="server"></asp:TextBox>

<asp:TextBox id="TextBox_subscribe" runat="server"></asp:TextBox>

<asp:Button id="Button_search" runat="server" Text="Search"></asp:Button>

<asp:Button id="Button_submit" runat="server" Text="Submit"></asp:Button>

<asp:RequiredFieldValidator id="RequiredFieldValidator1" runat="server" ErrorMessage="Email must not left blank" ControlToValidate="TextBox_subscribe"></asp:RequiredFieldValidator>

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I had 2 buttons with different purpose, but when I pressed the search button for textbox_search(in empty) which I never validate to it, message error on textbox_subscribe came out "email must not left blank".
So how to make RequiredFieldValidator only work with Button_submit not Button_search ?

Thanks in advance.