<asp:TextBox ID="TextBox1" runat="server" AutoPostBack = "true" CausesValidation = "true"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" ControlToValidate = "TextBox1"
ValidationExpression = "^(0?[1-9]|[12][0-9]|3[01])[\/\-](0?[1-9]|1[012])[\/\-]\d{4}$"
runat="server" ErrorMessage="Invalid Date format. Valid Date Format dd/MM/yyyy">
</asp:RegularExpressionValidator>
Validates dd/mm/yyyy format