string sample = "1a2b3c4d";
MatchCollection matches = Regex.Matches(sample, @"d");

List<string> matchesList = new List<string>();

foreach (Match match in matches)
    matchesList.Add(match.Value);

matchesList.ForEach(Console.WriteLine);

List<string> matchesList2 = new List<string>();

matchesList2.AddRange(
    from Match match in matches
    select match.Value
);

matchesList2.ForEach(Console.WriteLine);