Dim str As String, pos As Long
 
With RichTextBox1
   str = "bbbbbb"
   .Text = "aaaaa" & vbCrLf & str & vbCrLf & "cccccc"
   pos = .Find(str)
   If pos >= 0 Then
      .SelStart = pos
      .SelLength = Len(str) + 2
      .SelText = ""
   End If
End With