Ooooooh, I think I get it now.
[highlight=c#]
string plainText = richTextBox1.Text;
int firstPt = plainText.ToLower().IndexOf("msgbox.show \"") + "msgbox.show \"".Length;
int lastPt = plainText.ToLower().IndexOf("\"", firstPt);
MessageBox.Show(plainText.Substring(firstPt, lastPt - firstPt));
[/highlight]