using System;

using System.Text.RegularExpressions;

​

namespace ConsoleApp6 {

  public class Program {

    public static void Main()

    {

      String input = @"I dont like space "" "" in between "" "" sting ";

      var output= input.ReplaceInBetween('"', '"', ' ', string.Empty);

      Console.Write(output);

    }

  }

​

  public static class Extensions {

    public static string ReplaceInBetween(this String str,  char start, char end, char middle, string replacewith)

    {

       Regex x = new Regex($"([{start}])({middle})([{end}])");

      str=  x.Replace(str, "$1" + replacewith + "$3");

      return str;

    }

  }

}