using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var value =
				"super exemple of string key : text I want to keep - end of my string"
					.Between(
				        "key : ",
				        " - ");
			
		Console.WriteLine(value);
	}
}

public static class Ext
{
	static string Between(this System.String source, string left, string right)
	{
		return Regex.Match(
				source,
				string.Format("{0}(.*){1}", left, right))
			.Groups[1].Value;
	}
}