Validate date in code behind with regex – VB.NET

Code behind

'define public var
Protected StartDate As String
Protected EndDate As String

'define other var
Dim sDate = Request.Form("startDateTbx")
Dim eDate = Request.Form("endDateTbx")
Dim r As Regex 
r = New Regex("^(0[1-9]|[1-9]|1[012])/(0[1-9]|[1-9]|[12][0-9]|3[01])/20\d\d$") 

'run validation
If sDate = "" Then
     startDateLbl.Text = "Required"
 ElseIf Not r.Match(sDate).Success Then
    startDateLbl.Text = "Invalid date. Use mm/dd/yyyy."
 Else
    startDateLbl.Text = ""
End If

If eDate <> "" And Not r.Match(eDate).Success Then
   endDateLbl.Text = "Invalid date. Use mm/dd/yyyy."
Else
   endDateLbl.Text = ""
End If

StartDate = sDate
 EndDate = eDate


Markup page

<%=StartDate%>
<%=EndDate%>