How to return matched string all locations?

Question

My code is below
Dim a As String = "ABCD efabCd efcdfg"
Dim b As String = "cd"

    Dim i As Int32 = a.IndexOf(b, StringComparison.OrdinalIgnoreCase)  

There are 3 "cd" in the a string, how to return all index locations? I need to return "2, 9, 14", but the i only return "2".

Answer 1

    Dim a As String = "ABCD efabCd efcdfg"  
    Dim b As String = "cd"  

    Dim rx = New Regex(b, RegexOptions.Compiled Or RegexOptions.IgnoreCase)  
    Dim match As Match = rx.Match(a)  

    Do While match.Success  
        MessageBox.Show(match.Value & ": " & match.Index)  
        match = match.NextMatch()  
    Loop