using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = "[[:de:Hello World]]";

        var output = Regex.Replace(input, @"(\[\[:de:)(.+)(\]\])",  m => "{{de|" +  m.Groups[2].Value + "}}");

        Console.WriteLine(output);

    }

}