page is posted back even when an valid email is not entered

<div class="empLTitle">
                        Email</div>
                    <div class="empLTextBox">
                        <asp:TextBox ID="txtEmail" EnableViewState="false" CssClass="nonwatermarked" runat="server"></asp:TextBox><asp:RegularExpressionValidator
                            ID="expEmail" runat="server" ControlToValidate="txtEmail" ErrorMessage="valid email address required"
                            ValidationExpression="^([0-9a-zA-Z]([-.\w]*[0-9a-zA-Z])*@([0-9a-zA-Z][-\w]*[0-9a-zA-Z]\.)+[a-zA-Z]{2,9})$"></asp:RegularExpressionValidator></div>