using System;

using System.Text.RegularExpressions;

​

public class Example

{

    public static void Main()

    {

        string pattern = @"(?:[^-\n]+-){2}(.*)$";

        string input = @"

01:23:45 - site name1 - message received1

01:23:45 - site name2 - message received2

01:23:45 - site name3 - message received3

01:23:45 - site name4 - message received4";

        RegexOptions options = RegexOptions.Multiline | RegexOptions.IgnoreCase;

        foreach (Match m in Regex.Matches(input, pattern, options))

        {

            Console.WriteLine("{0}", m.Groups[1].Value);

        }

    }

}