using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string original = @"aaaaaaaaaa""aaaa""aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa""aaaaaaaaaa""aaaaaaaaaaaaa";

        foreach (Match match in Regex.Matches(original, @"\""+[A-z].*?\""", RegexOptions.IgnoreCase))

        {

            string valorEncontrado = match.Value.Replace("\"", String.Empty);

            string novoValor = new String('b', valorEncontrado.Length);

            original = original.Replace(match.Value, "\"" + novoValor + "\"");

        }

​

        Console.WriteLine(original);

    }

}