STM  709X915-YZZJ3    00052086-1A03 017371   
STM  700888X-80846    00006086-1A01 017371   
STM  70X8880-80826    00005086-1A01 017371 
weitere ca. 200 solcher Zeilen

Imports System.IO
 
Public Class Form1
 
    Sub DateiLesen(DateiName)
        Dim fs As FileStream = New FileStream( _
            DateiName, FileMode.OpenOrCreate, FileAccess.ReadWrite)
        Dim sr As StreamReader = New StreamReader(fs)
        sr.BaseStream.Seek(0, SeekOrigin.Begin)
        While sr.Peek() > -1
            ListBox1.Items.Add(sr.ReadLine())
        End While
        sr.Close()
        fs.Close()
    End Sub
 
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) _
            Handles Button1.Click
        DateiLesen("Pfad der txtDatei")
        Dim s As String = ListBox1.Items(0)
        TextBox1.Text = s.Substring(Len(s) - 5, 5)
    End Sub
 
End Class

'Bei bekannter Zeilenlänge ...
Dim startposition As Integer = 37
Dim length As Integer = 5
 
Using sr As New IO.StreamReader("F:\daten\demodaten1.txt")
    Dim substringtocheck As String = sr.ReadLine.Substring(startposition, _
      length)
    '...... 
End Using