using System;

using System.Text.RegularExpressions;

using Newtonsoft.Json;

​

public class Program

{

    public class Tmp

    {

        public string A {get;set;}

    }

    public static void Main()

    {

        var tmp = new Tmp()

        {

            A = "hello \"this\" is a test",

        };

        string input1 = JsonConvert.SerializeObject(tmp);

        Console.WriteLine(JsonStringify(input1));

    }

    static string JsonStringify(string input){

        var Matches = Regex.Matches(input, "\"[a-zA-Z]+\\s[a-zA-Z]+.*\"", RegexOptions.IgnoreCase);

        //Console.WriteLine(Matches.Count);

        foreach(Match match in Matches){

            //Console.WriteLine(match);

            input = Regex.Replace(input,match.Value,"--ReplaceLatter--"+match.Index);

        }

        //Remove all white space

        input = input.Replace(" ", String.Empty).Replace("\r", String.Empty).Replace("\n", String.Empty).Replace("\t", String.Empty);

        //locate the unmatched with real value

        foreach(Match match in Matches){

            input = Regex.Replace(input,"--ReplaceLatter--"+match.Index,match.Value);

        }   

        return input;

    }

}