Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
        Dim str As String
        Dim len, zeichenAnz As Integer
        Dim zeichen As Char
        Dim k As Boolean = True
 
        zeichenAnz = 1
        str = TextBox1.Text
        len = str.Length
 
        While k = True
 
            For i As Integer = 0 To len - 1
                zeichen = str.Chars(i)
                If zeichen = "$" And zeichenAnz Mod 2 = 0 Then
                    str = str.Insert(i + 1, "[\tex]")
                    str = str.Remove(i, 1)
                    zeichenAnz = zeichenAnz + 1
                    len = str.Length
                ElseIf zeichen = "$" And zeichenAnz Mod 2 <> 0 Then
                    str = str.Insert(i + 1, "[tex]")
                    str = str.Remove(i, 1)
                    zeichenAnz = zeichenAnz + 1
                    len = str.Length
                End If
            Next
 
            If Not str.IndexOf("$") >= 0 Then
                k = False
            End If
        End While
 
        TextBox1.Text= str
    End Sub