using System;

using System.Text.RegularExpressions;

public class Program

{

    public static Regex r = new Regex(@"```json(.*)```", RegexOptions.Singleline);

    public static void Main()

    {

        var test = @"

```json

{

    ""name"": ""Alice""

}

```";

        var captured = r.Match(test);

        Console.WriteLine($"Sample:\n{test}");

        Console.WriteLine($"Captured:\n{captured.Groups[1]}");

    }

}