// @nuget: Sprache -version 2.3.1

//https://stackoverflow.com/questions/51231883/sprache-cannot-recognise-this-sequence/70297981

using Sprache;

using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {   

       var text = "a11a11a";

       var n = text.Length;

       var grammer =

           from open in Parse.Letter.Once()

           from content in Parse.LetterOrDigit.Repeat(n - 2)

           from close in Parse.Letter.Once()

           select open.Concat(content).Concat(close);   

        var result= grammer.Parse(text);

        foreach (char c in result)

                Console.WriteLine(c);

    }

}

​