why does this regex result in two matches?

gistfile1.txt

public virtual string[] _ParseCsvFormat(string value)
        {
            //var regexstr = @"(?:(?:""(?<m>[^""]*?,[^""]*?)""|(?<m>[^,]*)))(?:,|$)";
            var regexstr = @"
(?:
  (?:
    ""
    (?<m>[^""]*?,[^""]*?)
    ""
    |
    (?<m>[^,]*)
  )
)
(?:,|$)    
";
            var regex = new Regex(regexstr, RegexOptions.IgnorePatternWhitespace);
            var matches = regex.Matches(value);
            return matches.Cast<Match>()
                .Select(m => m.Groups["m"].Value)
                .ToArray();
        }

_ParseCsvFormat("a"); //results in {"a",""} and I was hoping for {"a"}

I don't see from where the second match is coming.