Extract all strings between two strings

Here is a solution using RegEx. Don't forget to include the following using statement.

using System.Text.RegularExpressions

It will correctly return only text between the start and end strings given.

Will not be returned:

akslakhflkshdflhksdf

Will be returned:

FIRSTSTRING
SECONDSTRING
THIRDSTRING

It uses the regular expression pattern [start string].+?[end string]

The start and end strings are escaped in case they contain regular expression special characters.

    private static List<string> ExtractFromString(string source, string start, string end)
    {
        var results = new List<string>();

        string pattern = string.Format(
            "{0}({1}){2}", 
            Regex.Escape(start), 
            ".+?", 
             Regex.Escape(end));

        foreach (Match m in Regex.Matches(source, pattern))
        {
            results.Add(m.Groups[1].Value);
        }

        return results;
    }

You could make that into an extension method of String like this:

public static class StringExtensionMethods
{
    public static List<string> EverythingBetween(this string source, string start, string end)
    {
        var results = new List<string>();

        string pattern = string.Format(
            "{0}({1}){2}",
            Regex.Escape(start),
            ".+?",
             Regex.Escape(end));

        foreach (Match m in Regex.Matches(source, pattern))
        {
            results.Add(m.Groups[1].Value);
        }

        return results;
    }
}

Usage:

string source = "A1FIRSTSTRINGA2A1SECONDSTRINGA2akslakhflkshdflhksdfA1THIRDSTRINGA2";
string start = "A1";
string end = "A2";

List<string> results = source.EverythingBetween(start, end);

    private static List<string> ExtractFromBody(string body, string start, string end)
    {
        List<string> matched = new List<string>();

        int indexStart = 0;
        int indexEnd = 0;

        bool exit = false;
        while (!exit)
        {
            indexStart = body.IndexOf(start);

            if (indexStart != -1)
            {
                indexEnd = indexStart + body.Substring(indexStart).IndexOf(end);

                matched.Add(body.Substring(indexStart + start.Length, indexEnd - indexStart - start.Length));

                body = body.Substring(indexEnd + end.Length);
            }
            else
            {
                exit = true;
            }
        }

        return matched;
    }