string str = "email : avasfish@mail.ru бла бла";
            Regex regex = new Regex(@"email : ([\w.@]+)", RegexOptions.IgnoreCase);
            regex.GetGroupNames();

            MatchCollection matches = regex.Matches(str);

            foreach (Match match in matches)
            {
                GroupCollection groups = match.Groups;
                Console.WriteLine("значение '{0}' позиция {1}",
                                  groups[1].Value,
                                  groups[1].Index);
            }

            Console.WriteLine();
            Console.ReadLine();