Set ASP.Net RegularExpression Validator Expression based on DropDownList using C# and VB.Net

Hey rakibxl,

Please refer below smaple.

HTML

<asp:DropDownList runat="server" ID="ddlCountry" OnSelectedIndexChanged="Select">
    <asp:ListItem Text="Select" />
    <asp:ListItem Text="India" />
    <asp:ListItem Text="USA" />
    <asp:ListItem Text="UK" />
    <asp:ListItem Text="australia" />
</asp:DropDownList>
<asp:TextBox runat="server" ID="txtVatCode" />
<asp:RegularExpressionValidator ID="RegtxtVatCode" Enabled="false" runat="server"
    ControlToValidate="txtVatCode" Display="Dynamic" CssClass="help-block" ErrorMessage="WrongVatCode">
</asp:RegularExpressionValidator>
<asp:Button Text="Validate" runat="server" OnClick="Validate" CausesValidation="false" />

Code

C#

string VatRegularExpression = "";
protected void Select(object sender, EventArgs e)
{
    if (ddlCountry.SelectedIndex > 2)
    {
        VatRegularExpression = "^[I-T]";
    }
}

protected void Validate(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(VatRegularExpression))
    {
        RegtxtVatCode.Enabled = true;
        RegtxtVatCode.ValidationExpression = VatRegularExpression;
    }
    else
    {
        RegtxtVatCode.Enabled = false;
        RegtxtVatCode.ValidationExpression = VatRegularExpression;
    }
}

VB.Net

Private VatRegularExpression As String = ""
Protected Sub [Select](ByVal sender As Object, ByVal e As EventArgs)
    If ddlCountry.SelectedIndex > 2 Then
        VatRegularExpression = "^[I-T]"
    End If
End Sub
Protected Sub Validate(ByVal sender As Object, ByVal e As EventArgs)
    If Not String.IsNullOrEmpty(VatRegularExpression) Then
        RegtxtVatCode.Enabled = True
        RegtxtVatCode.ValidationExpression = VatRegularExpression
    Else
        RegtxtVatCode.Enabled = False
        RegtxtVatCode.ValidationExpression = VatRegularExpression
    End If
End Sub

Screenshot