Option Strict On
Option Explicit On
Option Infer Off
Public Class Form1
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        ListView1.Items.Clear()
        ListView1.Columns.Clear()
        ListView1.Columns.Add("Sequence")
        ListView1.Columns.Add("Indexes of occurrence")
        Dim sequences As List(Of Sequence) = DetectSequences("EDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGB")
        For Each s As Sequence In sequences
            Dim item As New ListViewItem(s.Sequence)
            item.Tag = s
            item.SubItems.Add(s.IndexesToString)
            ListView1.Items.Add(item)
        Next
        ListView1.AutoResizeColumns(ColumnHeaderAutoResizeStyle.HeaderSize)
    End Sub
    Function DetectSequences(s As String, Optional minLength As Integer = 5, Optional MaxLength As Integer = 8) As List(Of Sequence)
        Dim foundPatterns As New List(Of String)
        Dim foundSequences As New List(Of Sequence)
        Dim potentialPattern As String = String.Empty, potentialMatch As String = String.Empty
        For start As Integer = 0 To s.Length - 1
            For length As Integer = 1 To s.Length - start
                potentialPattern = s.Substring(start, length)
                If potentialPattern.Length < minLength Then Continue For
                If potentialPattern.Length > MaxLength Then Continue For
                If foundPatterns.IndexOf(potentialPattern) = -1 Then
                    foundPatterns.Add(potentialPattern)
                End If
            Next
        Next
        For Each pattern As String In foundPatterns
            Dim sequence As New Sequence With {.Sequence = pattern}
            For start As Integer = 0 To s.Length - pattern.Length
                Dim length As Integer = pattern.Length
                potentialMatch = s.Substring(start, length)
                If potentialMatch = pattern Then
                    sequence.Indexes.Add(start)
                End If
            Next
            If sequence.Indexes.Count > 1 Then foundSequences.Add(sequence)
        Next
        Return foundSequences
    End Function
    Public Class Sequence
        Public Sequence As String =""
        Public Indexes As New List(Of Integer)
        Public Function IndexesToString() As String
            Dim sb As New System.Text.StringBuilder
            For i As Integer = 0 To Indexes.Count - 1
                If i = Indexes.Count - 1 Then
                    sb.Append(Indexes(i).ToString)
                Else
                    sb.Append(Indexes(i).ToString &",")
                End If
            Next
            Return sb.ToString
        End Function
    End Class
    Private Sub ListView1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ListView1.SelectedIndexChanged
        If ListView1.SelectedItems.Count = 0 Then Exit Sub
        RichTextBox1.Clear()
        RichTextBox1.Text ="EDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGBEDCRFVTGB"
        Dim selectedSequence As Sequence = DirectCast(ListView1.SelectedItems(0).Tag, Sequence)
        For Each i As Integer In selectedSequence.Indexes
            RichTextBox1.SelectionStart = i
            RichTextBox1.SelectionLength = selectedSequence.Sequence.Length
            RichTextBox1.SelectionBackColor = Color.Red
        Next
    End Sub
End Class

Public Function DetermineSequence(ByVal strTarget As String) As String

    Dim strSequence As String = String.Empty

    Dim intLengthOfTarget As Integer = strTarget.Length

    'Check for a valid Target string.
    If intLengthOfTarget > 2 Then

        'Try 1/2 of Target, 1/3 of Target, 1/4 of Target, etc until sequence is found.
        Dim intCursor As Integer = 2

        Do Until strSequence.Length > 0 OrElse intCursor = intLengthOfTarget

            'Don't even test the string if its length is not a divisor (to an Integer) of the length of the target String.
            If IsDividendDivisibleByDivisor(strTarget.Length, intCursor) Then

                'Get the possible sequence.
                Dim strPossibleSequence As String = strTarget.Substring(0, (intLengthOfTarget / intCursor))

                'See if this possible sequence actually is the repeated String.
                If IsPossibleSequenceRepeatedThroughoutTarget(strPossibleSequence, strTarget) Then

                    'The repeated sequence has been found.
                    strSequence = strPossibleSequence

                End If

            End If

            intCursor += 1

        Loop

    End If

    Return strSequence

End Function

Private Function IsDividendDivisibleByDivisor(ByVal intDividend As Integer, ByVal intDivisor As Integer) As Boolean

    Dim bolDividendIsDivisbleByDivisor As Boolean = False

    Dim intOutput As Integer

    If Integer.TryParse((intDividend / intDivisor), intOutput) Then

        bolDividendIsDivisbleByDivisor = True

    End If

    Return bolDividendIsDivisbleByDivisor

End Function

Private Function IsPossibleSequenceRepeatedThroughoutTarget(ByVal strPossibleSequence As String, ByVal strTarget As String) As Boolean

    Dim bolPossibleSequenceIsRepeatedThroughoutTarget As Boolean = False

    Dim intLengthOfTarget As Integer = strTarget.Length
    Dim intLengthOfPossibleSequence As Integer = strPossibleSequence.Length

    Dim bolIndicatorThatPossibleSequenceIsCertainlyNotRepeated As Boolean = False

    Dim intCursor As Integer = 1

    Do Until (intCursor * intLengthOfPossibleSequence) = strTarget.Length OrElse bolIndicatorThatPossibleSequenceIsCertainlyNotRepeated

        If strTarget.Substring((intCursor * intLengthOfPossibleSequence), intLengthOfPossibleSequence) <> strPossibleSequence Then

            bolIndicatorThatPossibleSequenceIsCertainlyNotRepeated = True

        End If

        intCursor += 1

    Loop

    If Not bolIndicatorThatPossibleSequenceIsCertainlyNotRepeated Then

        bolPossibleSequenceIsRepeatedThroughoutTarget = True

    End If

    Return bolPossibleSequenceIsRepeatedThroughoutTarget

End Function

for each character index i
    for each character index after that j
        compare substring(i, j-i) to substring(j, j-i)
        if equal, record as a found repeating substring

public List<String> getRepetitions(String string) {
   List<String> repetitions = new ArrayList<String>();
   Map<String, List<Integer>> rep = new HashMap<String, List<Integer>>(), repOld;
   // init rep, add start position of all single character length strings
   for (int i = 0; i < string.length(); i++) {
      String s = string.substring(i, i + 1); // startIndex inclusive, endIndex exclusive
      if (rep.containsKey(s)) {
         rep.get(s).add(new Integer(i));
      } else {
         List<Integer> l = new ArrayList<Integer>();
         l.add(new Integer(i));
         rep.put(l);
      }
   }
   // eliminate those with no repetitions and add the others to the solution
   for (Map.Entry<String, Integer> e : rep.entrySet()) {
      if (e.getValue().size() < 2) {
         rep.remove(e.getKey());
      } else {
         repetitions.add(e.getKey());
      }
   }
   for (int len = 1; rep.size() > 0; len++) {
      repOld = rep;
      rep = new HashMap<String, List<Integer>>();
      for (Map.EntrySet<String, List<Integer>> e : repOld.entrySet()) {
         for (Integer i : e.getValue()) { // for all start indices
            if (i.intValue() + len + 1 >= string.length())
               break;
            String s = e.getKey() + string.charAt(i.intValue() + len + 1);
            if (rep.containsKey(s)) {
               rep.get(s).add(i);
            } else {
               List<Integer> l = new ArrayList<Integer>();
               l.add(i);
               rep.put(l);
            }
         }
      }
      // eliminate repetitions and add to solution
      for (Map.Entry<String, Integer> e : rep.entrySet()) {
         if (e.getValue().size() < 2) {
            rep.remove(e.getKey());
         } else {
            repetitions.add(e.getKey());
         }
      }
   }
   return repetitions; // ordered by length, so last = longest
}