How can i extract a return string from a Regex Match in C#? [duplicate]

Question

I would like to extract from a Regex Match a string return. The code is that:

public static string timeConversion(string s)
        {
            CultureInfo converter = new CultureInfo("pt-BR");
            var conv = DateTime.Parse(s, converter).ToString();
            var reg = new Regex("[0-9]{2}:?[0-9]{2}:?[0-9]{2}");
            var regRes = reg.Matches(conv);
            return (i don't know)
        }

I wish to return a string with the value, for example: "21:53:09"