Dim rtbLines2() As String = WTEdit.RichTextBox1.Lines
        Dim redi As Integer
        For redi = 0 To rtbLines2.GetUpperBound(0)
            If rtbLines2(redi).Contains("Achtung") Then
                WTEdit.RichTextBox1.SelectionStart = _
                  WTEdit.RichTextBox1.GetFirstCharIndexFromLine(redi)
                WTEdit.RichTextBox1.SelectionLength = rtbLines2(redi).Length
                WTEdit.RichTextBox1.SelectionColor = Color.Red
                WTEdit.RichTextBox1.SelectionFont = New Font( _
                  WTEdit.RichTextBox1.Font, FontStyle.Bold)
            End If
        Next
    End Sub

Private Sub RichTextBox1_KeyDown(sender As Object, e As KeyEventArgs) Handles _
  RichTextBox1.KeyDown
    RichTextBox1.SelectionColor = Color.Black
End Sub

    Private Sub Button1_Click_1(sender As Object, e As EventArgs) Handles _
      FTUndo.Click
        If Me.RichTextBox1.CanUndo Then
            Me.RichTextBox1.Undo()
        End If
    End Sub
 
    Private Sub FTWdh_Click(sender As Object, e As EventArgs) Handles _
      FTWdh.Click
        If Me.RichTextBox1.CanRedo Then
            Me.RichTextBox1.Redo()
        End If
    End Sub