我可以向后浏览文本以在Visual C＃中查找字符串吗？

我有一个搜索功能，可以从文本的开头到结尾在Richtextbox中搜索一个字符串。当我找到最后一个符合条件时，它从头开始。

这里的代码：

       #region Search

      private void txtSearch_KeyPress(object sender, KeyPressEventArgs e)
    {
        start = 0;
        end = 0;
    }     

    //Searchfield
    private void toolStripTextBoxSearch_Click(object sender, EventArgs e)
    {

    }//end TextBoxSearch

    public int FindMyText(string txtToSearch, int searchStart, int searchEnd)
    {
        // Set the return value to -1 by default.
        int retVal = -1;

        // A valid starting index should be specified.
        if (searchStart >= 0)
        {
            // A valid ending index
            if (searchEnd > searchStart || searchEnd == -1)
            {
                // Find the position of search string in RichTextBox
                indexOfSearchText = richTextBox.Find(txtToSearch, searchStart, searchEnd, RichTextBoxFinds.None);
                // Determine whether the text was found in richTextBox1.
                if (indexOfSearchText != -1)
                {
                    // Return the index to the specified search text.
                    retVal = indexOfSearchText;
                }
            }
        }
        return retVal;
    }//end FindMyText

    private void buttonSearch_left_Click(object sender, EventArgs e)
    {

    }

    private void buttonSearch_right_Click(object sender, EventArgs e)
    {
        int startindex = 0;

        if (txtSearch.Text.Length > 0)
        {
             startindex = FindMyText(txtSearch.Text, start, richTextBox.Text.Length);
        }
            // If string was not found report it
            if (startindex < 0 )
            {
                if (stringfoundflag==1)
                {
                startindex = FindMyText(txtSearch.Text, 0, richTextBox.Text.Length); //Start at Pos. 0
                }
                else
                {
                    MessageBox.Show("Not found in Textfield", "Search", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
            }//end if 

             if (startindex >= 0)
            {
                stringfoundflag = 1;
            // Set the highlight color as red
            //richTextBox.SelectionColor = Color.Red;
            // Find the end index. End Index = number of characters in textbox
            int endindex = txtSearch.Text.Length;
                // Highlight the search string
                richTextBox.Select(startindex, endindex);
                // mark the start position after the position of
                // last search string
                start = startindex + endindex;
        }
    }

    // Reset the richtextbox when user changes the search string
    private void textBox1_TextChanged(object sender, EventArgs e)
    {
        start = 0;
        indexOfSearchText = 0;
    }

    private void txtSearch_Enter(object sender, EventArgs e)
    {
        if (txtSearch.Text == "Search")
        {
            txtSearch.Text = "";
        }
    }

    private void txtSearch_Leave(object sender, EventArgs e)
    {
        if (txtSearch.Text == "")
        {
            txtSearch.Text = "Search";
        }
    }

    #endregion

我有两个按钮，一是向右搜索，一是向左搜索，右边的功能就像开始时的描述一样。

问题：

我可以使用解决方案从右向左反转搜索功能，还是必须更改整个搜索功能？

示例：从第一个一致开始，然后跳到最后一个一致，然后从第二个跳到最后一个，依此类推。