在RichTextBox中添加更多文本并突出显示时，它将清除所有以前的突出显示

Question

I'm making a debug/log form that saves all the things done in the others forms at the specific time: like pressing a button, entering some info in textbox, etc.

In every line I highlight the actual time that corresponds to the same day, for example, today is 10/08/2019 so:

Example 1

10/08/2019 corresponds to the actual date, so it's highlighted. The problem is that when I do another thing like pressing L button it shows like this:

Example 2

Below I put the code that it interprets this: CMD = RichTextBox

private void ChequearDatos()
{
    string line = CMD.Text;
    int x = xk, xx = 0, lent = 0;
    lent = line.Length;

    do
    {
        else if (line[x] == '\n')
        {
            xk = x;                   
            x++;
        }

        else if (line[x] == '■')
        {
            xx = x + 1;
            do
            {
                xx = xx + 1;
            }
            while (line[xx] != '=');
            string pedazo = line.Substring(x + 2, (xx - x) - 12);
            if (pedazo == Convert.ToString(DateTime.Today.Day + "/" + DateTime.Today.Month + "/" + DateTime.Today.Year))
            {
                CMD.SelectionStart = x;
                CMD.SelectionLength = xx - x + 1;
                CMD.SelectionColor = System.Drawing.Color.OrangeRed;
            }
            else
            {
                CMD.SelectionStart = x;
                CMD.SelectionLength = xx - x + 1;
                CMD.SelectionColor = System.Drawing.Color.DarkKhaki;
            }
            CMD.SelectionStart = CMD.TextLength;

            CMD.ScrollToCaret();
            if (xx + 1 > lent) { break; }
            else { x = xx + 1; } 
        }
        else { x = x + 1; }
    }
    while (x <= lent - 1);
}

The program search for ■ and =, the inside of it if it's the same date it will be highlighted with DarkKhaki colour, if not, it will be highlighted with OrangeRed colour.

In the first run, it runs good, but when I add more text and call that function again it highlight everything in white except the last message.

Edit: I tried without saving the xk int variable, but when I call this function the program needs to process all the text again and starts to blink until checked all of it.

Answer 1

I fixed it appending it like @TnTinMin said, before I call this function another function writes in the RTB as below:

void PrintRTB(string a)
{
  RTB.text += "■ " +  a + "\n";
  ChequearDatos();
}

Now I use the command [.appendtext] and works!

void PrintRTB(string a)
{
  RTB.AppendText("■ " + a + "\n");
  ChequearDatos();
}