using System;

using System.Globalization; 

using System.Text.RegularExpressions;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string input = "log_{yyyy-MM} foo {HH}.txt";

        DateTime now = DateTime.Now;

​

        string[] results = Regex.Matches(input,@"\{.*?\}" ).Cast<Match>().Select(m => m.Value).ToArray(); 

​

​

        foreach(string result in results) {

            input = input.Replace(result,now.ToString(result));

        }

​

        Console.WriteLine(string.Format("log_{0} foo {1}.txt",now.ToString("yyyy-MM"),now.ToString("HH")));

    }

}