You could try this:
int charIndex = 0;
foreach (string sline in rtb.Lines)
{
label1.Text= sline;
label2.Text = label1.Text.IndexOf("mk").ToString();
charIndex += sline.IndexOf("mk");
rtb.Select(charIndex, "mk".Length);
charIndex += sline.Length - sline.IndexOf("mk") + 1;
}