RichTextBox 查找内容并红色显示

RichTextBox 中打开文件并显示，按查找，匹配所有相同的文字并红色显示：

private void BtnSearch_Click(object sender, EventArgs e)
        {
            string strKeyWords = txtKeyWords.Text.Trim();
            //int intFirst = rtxtbox.Text.IndexOf(strKeyWords);
            //if (intFirst >= 0)
            //{
            //    rtxtbox.Select(intFirst, strKeyWords.Length );
            //    rtxtbox.SelectionColor = Color.Red;
            //}
            Regex regex = new Regex(strKeyWords);
            Match match = regex.Match(rtxtbox.Text);
            while (match.Success)
            {
                rtxtbox.Select(match.Index, strKeyWords.Length);
                rtxtbox.SelectionColor = Color.Red;
                match = regex.Match(rtxtbox.Text, match.Index + strKeyWords.Length);
            }
        }