How to avoid 'show summary' display while using regular exp validation

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ASP.NET

In one of my page I use regular expression validation for DDL's. Though it works fine with 'Show messageBox=true' it also displays the same error message outside the message box. Hereunder is my code:

<asp:RegularExpressionValidator ID="RegularExpressionValidator13" runat="server"
            ControlToValidate="DropDownList8" ErrorMessage="Select Scheme"
            style="" ValidationExpression="[a-z0-9_]{1,45}" Font-Size="X-Small"></asp:RegularExpressionValidator>

<asp:ValidationSummary ID="ValidationSummary1" runat="server"
            ShowMessageBox="True"  ShowSummary="false" />

Can anyone help me out?