Handling colorized Text in RichTextBox is buggy moving back with the cursor

Hello,

if the user enters a filter string in my RichTextBox I want the string OR + AND to be colorized.

Below code works until the user goes back with the cursor in order to delete/change code. Then some chars after the cursor gets automatically selected and overwritten, what I do not want. How to change this behaviour, any idea?

public Regex keyWords = new Regex("AND|OR");
 
        private void rowfilterRichTextBox_TextChanged(object sender, EventArgs e)
        {
            int selPos = rowfilterRichTextBox.SelectionStart;
            foreach (Match keyWordMatch in keyWords.Matches(rowfilterRichTextBox.Text))
            {
                rowfilterRichTextBox.Select(keyWordMatch.Index, keyWordMatch.Length);
                rowfilterRichTextBox.SelectionColor = Color.Blue;
                rowfilterRichTextBox.SelectionStart = selPos;
                rowfilterRichTextBox.SelectionColor = Color.Black;
            }
        }

Select allOpen in new window