Das [u]ist ein Beispiel.

Public Class Form1
 
    Private Sub Button6_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button6.Click
        If rtbEdit.SelectionFont IsNot Nothing Then
            Dim currentFont As System.Drawing.Font = rtbEdit.SelectionFont
            Dim newFontStyle As System.Drawing.FontStyle
 
            If rtbEdit.SelectionFont.Bold = True Then
                newFontStyle = FontStyle.Regular
                TextBox1.AppendText("[./b]")
                rtbEdit.Focus()
            Else
                newFontStyle = FontStyle.Bold
                TextBox1.AppendText("[.b]")
                rtbEdit.Focus()
            End If
 
            rtbEdit.SelectionFont = New Font( _
               currentFont.FontFamily, _
               currentFont.Size, _
               newFontStyle _
            )
        End If
    End Sub
 
 
End Class