Regular expression to validate Date

If you are using calendar control you can do this as :

protected void Calendar1_DayRender(object sender, DayRenderEventArgs e)
  {
      if (e.Day.Date <= DateTime.Today.Date)
      {
          e.Cell.Enabled= false; // Changes the Cells Enable Property
          e.Day.IsSelectable = false; // Disables the Cells Click Property
      }
  }

for more refer :http://www.dotnetfunda.com/articles/article600-customising-the-calendar-control-in-aspnet.aspx[^]


or while using a Ajax Calendar Extender control you can do as something explained here :
https://forums.asp.net/t/1615792.aspx?CalendarExtender+date+checker[^]