<asp:TextBox ID="txtTransitDays" runat="server"></asp:TextBox>

<asp:RegularExpressionValidator ID="TransitDaysRegexValidator"

     ControlToValidate="txtTransitDays" runat="server"

     ErrorMessage="Only positive integer numbers are allowed"

     ValidationExpression="^[1-9]\d*$" Display="Dynamic">

</asp:RegularExpressionValidator>

<asp:RequiredFieldValidator ID="TransitDayRequiredValidator"

     ControlToValidate="txtTransitDays"

     ErrorMessage="Required field" runat="server" Display="Dynamic">
</asp:RequiredFieldValidator>