dynamically add validation expression to RegularExpressionValidato control

Hi
i want to validate date format in textbox through regulrexpressionvalidator control

but i want to check  ValidationExpression from my clsdate class this public variable.

  Public  DateFormat As String = "dd/MM/yyyy"


<asp:RegularExpressionValidator ID="revDOE1" runat="server"

ControlToValidate="txtlFollowUp_date"                                            ErrorMessage="Wrong Date Format" ValidationExpression="^(((0[1-9]|[12]\d|3[01])\/(0[13578]|1[02])\/((1[6-9]|[2-9]\d)\d{2}))|((0[1-9]|[12]\d|30)\/(0[13456789]|1[012])\/((1[6-9]|[2-9]\d)\d{2}))|((0[1-9]|1\d|2[0-8])\/02\/((1[6-9]|[2-9]\d)\d{2}))|(29\/02\/((1[6-9]|[2-9]\d)(0[48]|[2468][048]|[13 ...