Real number parser based on http://blog.leifbattermann.de/2015/11/23/functional-monadic-parsers-ported-to-c/

RealNumberParser.cs

public static readonly Parser<char> Dot = CharP('.');
public static Parser<char> Sign = Sat(c => "+-".Contains(c));
public static Parser<char> Exponent = Sat(c => "Ee".Contains(c));

public static Parser<double> RealNumber() {
    return 
        from integer in Natural
        from fractional in (from dot in Dot
                    from digit in Natural
                            from r in Return(digit)
                    select r).Choice(Parsers.Return(0))
        from exponent in (from e in Exponent
                          from sign in Sign.Choice(Parsers.Return('+'))
                          from v in Natural
                          from r in Return(double.Parse($"{sign}{v}"))
                          select r).Choice(Parsers.Return(0d))
        from result in Return(double.Parse($"{integer}.{fractional}e{exponent}"))
        select result;
}