detect escaped json-string, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using Newtonsoft.Json;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            string str = "Hello, world!'\"\r\n()!";
            string json = JsonConvert.SerializeObject(str);
            Console.WriteLine("str: "+str);
            Console.WriteLine("JsonString: "+json);
            string str2 = JsonConvert.DeserializeObject<string>(json);
            Console.WriteLine("str2: "+str2);
            Console.WriteLine("strings equals? (str== str2): "+(str==str2));
            
            string str3 = "";
            
            //str
            str3 = str;
            Console.WriteLine( "str3 escaped? " + ( (str3[0]=='"') && (str3[str3.Length-1]=='"') ) );
            
            //json
            str3 = json;
            Console.WriteLine( "str3 escaped? " + ( (str3[0]=='"') && (str3[str3.Length-1]=='"') ) );
            
            
            
            
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,31 sec, absolute running time: 0,53 sec, cpu time: 0,3 sec, average memory usage: 25 Mb, average nr of threads: 5, absolute service time: 0,88 sec