select random from target string VB.NET - vb.net

Question

how could i make that
Source_Source.Text (richtextbox) :
src="test.com" akalsjdjalksdv src="another.com" asdbnmslkbjcxv
asdas as danms d amsn asdasd src="cold.com"asdas as dasd amnbs dma d sdf kjhds f src="find.com" asd kja sdasjhk d asdsrc="other.com" a jksdh asksjd hasdjh src="found.com"
what if i wanna get random src=" ", for example, if i clicked butten will show message = src="test.com" , another time if i clicked button will show another random, for example src="another.com" ,
my currently code only select first string which = src="test.com" , and i wanna select randoms src="[random test / cold / other / found / find]"
for example my next click can show message for src="find.com" that for example.
my code :
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim sSource As String = Source_Source.Text 'String that is being searched
Dim sDelimStart As String = Search_1.Text 'First delimiting word
Dim sDelimEnd As String = Search_2.Text 'Second delimiting word
Dim nIndexStart As Integer = sSource.IndexOf(sDelimStart) 'Find the first occurrence of f1
Dim nIndexEnd As Integer = sSource.IndexOf(sDelimEnd, nIndexStart + sDelimStart.Length + 1) 'Find the first occurrence of f2
If nIndexStart > -1 AndAlso nIndexEnd > -1 Then '-1 means the word was not found.
Dim res As String = Strings.Mid(sSource, nIndexStart + sDelimStart.Length + 1, nIndexEnd - nIndexStart - sDelimStart.Length) 'Crop the text between
MessageBox.Show(res) 'Display
Else
MessageBox.Show("One or both of the delimiting words were not found!")
End If
End Sub

Answer 1

Private Function RndUrl() As String
Dim UrlStr() As String = {"whatever.com", "whatever2.com"}
Return Rand.Next(0, UrlStr.Length) '"Rand" Declared at Class scope as Random
End Function
Usage:
TextBox1.Text = "src=""" & RndUrl() & ""

Answer 2

If I understand your question correctly, you're looking for a certain phrase in a String? If so look into String.Contains() method.
For example:
If Source_Source.Text.Contains(Search_1.Text) Then
' Do you're logic here if found
Source_Source.Text.SubString(0, 5) ' Or where ever that part of the string is that you want.
End If