using System;

using System.Linq;

using System.Collections.Generic;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = @"<Hello> [World]!";

        var regex = new Regex(@"(?<=\<).*(?=\>)|(?<=\[).*(?=\])");

        var index = 0;

        var replacements = new List<string>();

        var formatString = regex.Replace(input, (m) => 

        {

            replacements.Add(m.Value);

            return String.Format("{{{0}}}", index++);

        });

        Console.WriteLine(formatString);

        Console.WriteLine(String.Join(Environment.NewLine, replacements.Select((v, i) => i + ": " + v).ToArray()));

    }

}

​