Remove Last character from string if Special Character using C# .Net

Here I have created sample that will help you out.

HTML

<div>
    Enter Value:
    <asp:TextBox ID="txtValue" runat="server" />
    <br />
    <br />
    <asp:Button Text="Ok" runat="server" OnClick="GetResult" />
    <br />
    Result :
    <asp:Label ID="lblResult" runat="server" />
</div>

Code

protected void GetResult(object sender, EventArgs e)
{
    string value = txtValue.Text.Trim();
    lblResult.Text = RemoveSpecialCharacters(value);
}

private string RemoveSpecialCharacters(string str)
{
    StringBuilder sb = new StringBuilder();
    foreach (char c in str)
    {
        if ((c >= '0' && c <= '9') || (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == 32 || c == ',')
        {
            sb.Append(c);
        }
    }
    return sb.ToString();
}

Screenshot

Its removing from whole string i had the regex for this but i want to remove only from end

Something like this

lineone.Trim().ToLower().TrimEnd('?', '.', ',', ';', '!', ':'),