Task: Text between
C#: using System;

namespace TextBetween {
    class Program {
        static string TextBetween(string source, string beg, string end) {
            int startIndex;

            if (beg == "start") {
                startIndex = 0;
            }
            else {
                startIndex = source.IndexOf(beg);
                if (startIndex < 0) {
                    return "";
                }
                startIndex += beg.Length;
            }

            int endIndex = source.IndexOf(end, startIndex);
            if (endIndex < 0 || end == "end") {
                return source.Substring(startIndex);
            }
            return source.Substring(startIndex, endIndex - startIndex);
        }

        static void Print(string s, string b, string e) {
            Console.WriteLine("text: '{0}'", s);
            Console.WriteLine("start: '{0}'", b);
            Console.WriteLine("end: '{0}'", e);
            Console.WriteLine("result: '{0}'", TextBetween(s, b, e));
            Console.WriteLine();
        }

        static void Main(string[] args) {
            Print("Hello Rosetta Code world", "Hello ", " world");
            Print("Hello Rosetta Code world", "start", " world");
            Print("Hello Rosetta Code world", "Hello ", "end");
            Print("</div><div style=\"chinese\">你好嗎</div>", "<div style=\"chinese\">", "</div>");
            Print("<text>Hello <span>Rosetta Code</span> world</text><table style=\"myTable\">", "<text>", "<table>");
            Print("<table style=\"myTable\"><tr><td>hello world</td></tr></table>", "<table>", "</table>");
            Print("The quick brown fox jumps over the lazy other fox", "quick ", " fox");
            Print("One fish two fish red fish blue fish", "fish ", " red");
            Print("FooBarBazFooBuxQuux", "Foo", "Foo");
        }
    }
}

VB: Option Explicit

Private Const STRING_START As String = "Start"
Private Const STRING_END As String = "End"

Sub Main()
Dim Text As String, First As String, Last As String, Output As String

    Text = "Hello Rosetta Code world"
        First = "Hello "
            Last = " world"
    Output = "1- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "Hello Rosetta Code world"
        First = "Start"
            Last = " world"
    Output = Output & "2- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "Hello Rosetta Code world"
        First = "Hello "
            Last = "End"
    Output = Output & "3- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "</div><div style=\""chinese\"">你好嗎</div>..."
        First = "<div style=\""chinese\"">"
            Last = "</div>"
    Output = Output & "4- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "<text>Hello <span>Rosetta Code</span> world</text><table style=\""myTable\"">"
        First = "<text>"
            Last = "<table>"
    Output = Output & "5- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "<table style=\""myTable\""><tr><td>hello world</td></tr></table>"
        First = "<table>"
            Last = "</table>"
    Output = Output & "6- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "The quick brown fox jumps over the lazy other fox"
        First = "quick "
            Last = " fox"
    Output = Output & "7- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "One fish two fish red fish blue fish"
        First = "fish "
            Last = " red"
    Output = Output & "8- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "FooBarBazFooBuxQuux"
        First = "Foo"
            Last = "Foo"
    Output = Output & "9- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "</div><div style=\""chinese\"">你好嗎..."
        First = "<div style=\""chinese\"">"
            Last = "</div>"
    Output = Output & "10- " & Text_Between(Text, First, Last) & vbCrLf

    Text = ""
        First = "Start"
            Last = "End"
    Output = Output & "11- " & Text_Between(Text, First, Last) & vbCrLf

    Text = "Hello Rosetta Code world"
        First = "Start"
            Last = "End"
    Output = Output & "12- " & Text_Between(Text, First, Last)

    Debug.Print Output
End Sub

Private Function Text_Between(T$, F$, L$) As String
Dim i As Long
    i = InStr(T, L) + 1
    Select Case True
        Case T = "", F = "", InStr(T, F) = 0 And F <> STRING_START
            Text_Between = ""
        Case F = STRING_START And L = STRING_END
            Text_Between = T
        Case F = STRING_START
            Text_Between = Mid(T, 1, InStr(T, L) - 1)
        Case L = STRING_END, InStr(T, L) = 0, InStr(T, L) < InStr(T, F) And InStr(i, T, L) = 0
            Text_Between = Mid(T, Len(F) + InStr(T, F))
        Case F = L
            Text_Between = Mid(T, Len(F) + InStr(T, F), InStr(i, T, F) - Len(F) - 1)
        Case Else
            Text_Between = Mid(T, Len(F) + InStr(T, F), InStr(InStr(T, F), T, L) - (Len(F) + InStr(T, F)))
    End Select
End Function