using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Regex rx = new Regex(@"(?<!:):(?!:)(.+?)$", RegexOptions.Multiline);

​

        // Define a test string.        

        string text = "blah % blah :stuff\nblah %blah% blah\nblah %blah% blah\nblah %% blah\nblah blah % ::whatever";

        // Find matches.

        MatchCollection matches = rx.Matches(text);

​

        // Report the number of matches found.

        Console.WriteLine("{0} matches found in:\n   {1}", matches.Count, text);

            // Report on each match.

        foreach (Match match in matches)

        {

            GroupCollection groups = match.Groups;

            Console.WriteLine("'{0}' repeated at positions {1} and {2}", groups[0].Value, groups[0].Index, groups[1].Index);

        }

    }

}