using System;

using System.Globalization;

using Newtonsoft.Json;

​

public class Program

{

    public class RealDepositWalletData

    {

        [JsonProperty("dob")]

        public string BirthDate { get; set; }

    }

    public static void Main()

    {

        var input = new RealDepositWalletData { BirthDate = "30/11/2021", };

        var jsonSettings = new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };

        var json = JsonConvert.SerializeObject(input, jsonSettings);

        const string dobJsonPattern = @"""dob"":""(?<dd>\d+)\\*/(?<mm>\d+)\\*/(?<yy>\d+)""";

        const string dobReplacement = "\"dob\":\"${dd}\\/${mm}\\/${yy}\"";

        var formatted = System.Text.RegularExpressions.Regex.Replace(json, dobJsonPattern, dobReplacement);

        System.Console.WriteLine(json);

        System.Console.WriteLine(formatted);

    }

}