foreach (Match match in Regex.Matches(richTextBox1.Text, @"\[b\](.*?)\[/b\]", RegexOptions.IgnoreCase).OfType<Match>().Reverse())
{
richTextBox1.Select(match.Index, match.Length);
Console.WriteLine(match.Groups[0]);
richTextBox1.SelectionFont = new Font("Tahoma", 9, FontStyle.Bold);
richTextBox1.SelectedText = match.Groups[1].Value;
}