when i input wrong date in textbox then Regular Expression Validator get error but i want press button then not work . so how to solve it???

<asp:TextBox ID="txtstartdate" runat="server" Style="width: 305px" placeholder="dd/mm/yyyy"></asp:TextBox>
<asp:RegularExpressionValidator ID="birthdatecheck" runat="server" ValidationGroup="loan"
    ErrorMessage="*" ToolTip="Please enter date in dd/mm/yyyy format" Display="Dynamic"
    ForeColor="Red" Font-Bold="true" ControlToValidate="txtstartdate" ValidationExpression="(0[1-9]|[12][0-9]|3[01])/(0[1-9]|1[012])/\d{4}"></asp:RegularExpressionValidator>
<asp:Button ID="btnsaveauto" runat="server" CssClass="btn" Text="Save" OnClick="btnSave_Click"
    OnClientClick="return chackroot();" />