void Main()
{
    ParseAndPrintChallenge(@"Bearer realm=""FooCorp"", error=invalid_token, error_description=""The access token has expired""");
}

void ParseAndPrintChallenge(string input)
{
    var challenge = Grammar.Challenge.Parse(input);
    Console.WriteLine($"Scheme: {challenge.Scheme}");
    Console.WriteLine($"Parameters:");
    foreach (var p in challenge.Parameters)
    {
        Console.WriteLine($"- {p.Name} = {p.Value}");
    }
}

static class Grammar
{
    private static readonly Parser<char> DoubleQuote = Parse.Char('"');
    private static readonly Parser<char> EqualSign = Parse.Char('=');
    private static readonly Parser<char> Backslash = Parse.Char('\\');
    private static readonly Parser<char> Comma = Parse.Char(',');

    private static readonly Parser<char> SeparatorChar =
        Parse.Chars("()<>@,;:\\\"/[]?={} \t");

    private static readonly Parser<char> ControlChar =
        Parse.Char(Char.IsControl, "Control character");

    private static readonly Parser<char> TokenChar =
        Parse.AnyChar
            .Except(SeparatorChar)
            .Except(ControlChar);

    private static readonly Parser<string> Token =
        TokenChar.AtLeastOnce().Text();

    private static readonly Parser<char> QdText =
        Parse.AnyChar.Except(DoubleQuote);

    private static readonly Parser<char> QuotedPair =
        from _ in Backslash
        from c in Parse.AnyChar
        select c;

    private static readonly Parser<string> QuotedString =
        from open in DoubleQuote
        from text in QuotedPair.Or(QdText).Many().Text()
        from close in DoubleQuote
        select text;

    private static readonly Parser<string> ParameterValue =
        Token.Or(QuotedString);

    private static readonly Parser<Parameter> Parameter =
        from name in Token
        from _ in EqualSign
        from value in ParameterValue
        select new Parameter(name, value);

    private static readonly Parser<char> ListDelimiter =
        from leading in Parse.WhiteSpace.Many()
        from c in Comma
        from trailing in Parse.WhiteSpace.Or(Comma).Many()
        select c;

    private static readonly Parser<Parameter[]> Parameters =
        from p in Parameter.DelimitedBy(ListDelimiter)
        select p.ToArray();

    public static readonly Parser<Challenge> Challenge =
        from scheme in Token
        from _ in Parse.WhiteSpace.AtLeastOnce()
        from parameters in Parameters
        select new Challenge(scheme, parameters);
}

class Parameter
{
    public Parameter(string name, string value)
    {
        Name = name;
        Value = value;
    }

    public string Name { get; }
    public string Value { get; }
}

class Challenge
{
    public Challenge(string scheme, Parameter[] parameters)
    {
        Scheme = scheme;
        Parameters = parameters;
    }
    public string Scheme { get; }
    public Parameter[] Parameters { get; }
}