private void button3_Click(object sender, EventArgs e)
        {
            var rg = new Regex(@"^Client.*_delete\.xlsx$", RegexOptions.CultureInvariant | RegexOptions.IgnoreCase);
            var arr = new string[] {"Clienteewrwe_delete.xlsx", "Client_delete.xlsx", "Client_update.xlsx"};

            foreach (string s in arr)
            {
                Match match = rg.Match(s);

                if (match.Success)
                    richTextBox1.Text += match.Value + Environment.NewLine;
            }
        }