How to replace a string between two strings VB.Net

Not the nicest solution of filtering out the interesting part, but it works:

Dim newParam As String = "ACK"
Dim test As String = "MDSP:TSK:Demo:MDSR:Demo:::LAT:Test:P:2.3:"
Dim nStartExchange As Integer = test.IndexOf("::") + 3
Dim res As String = test.Substring(nStartExchange, test.Length - nStartExchange)
Dim nEndExchange As Integer = res.IndexOf(":"C) + nStartExchange
Dim result As String = test.Substring(0, nStartExchange) & newParam & test.Substring(nEndExchange)
MessageBox.Show(result)

I think you could also try using regular expressions in this case, but the above code works quite well for the string that you wanted to modify.

Another possible solution:

Dim newParam As String = "ACK"
Dim test As String = "MDSP:TSK:Demo:MDSR:Demo:::LAT:Test:P:2.3:"
Dim results As String() = test.Split(":"C)
Dim final As String = ""
results(7) = newParam
For i As Integer = 0 To results.Length - 1
    final += results(i)
    If i <> results.Length - 1 Then
        final += ":"
    End If
Next
MessageBox.Show(final)

Hope it helps!

1. simply use replace function,

Dim str = "MDSP:TSK:Demo:MDSR:Demo:::LAT:Test:P:2.3:"
str = str.Replace("LAT","ACK")

2. if u want to change string between 7th ":" & 8th ":" (Incase "LAT" word is not fixed for replacement."

Dim str = "MDSP:TSK:Demo:MDSR:Demo:::LAT:Test:P:2.3:"
               Dim colon_start_index = 0
               Dim colon_start_Occurance = 0
               For o As Integer = 0 To str.LastIndexOf(":") - 1
                   If colon_start_Occurance = 7 Then
                       Exit For
                   Else
                       colon_start_index = str.IndexOf(":", colon_start_index) + 1
                       colon_start_Occurance += 1
                   End If
               Next
               str = str.Replace(str.Substring(colon_start_index, str.IndexOf(":", colon_start_index) - (colon_start_index)), "ACK")

Happy Coding!
:)