Public Class Form1
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        Dim actChar, nextChar As String
        Dim actPos, nextPos As Integer
        Dim ausgabe As String = ""
        Dim running As Boolean = True
        Dim source As String = "AAABBBABBBAAABAAABBB".ToLower
        actPos = 0
        Do
            If source.Substring(actPos, 1) = "a" Then
                actChar = "a"
                nextChar = "b"
            Else
                actChar = "b"
                nextChar = "a"
            End If
            nextPos = source.IndexOf(nextChar, actPos)
            If nextPos = -1 Then
                running = False
                nextPos = source.Length
            End If
            Dim steps As Integer = nextPos - actPos
            ausgabe += steps.ToString + actChar.ToUpper
            actPos = nextPos
            Stop
        Loop While running = True
        MessageBox.Show(ausgabe)
    End Sub
End Class