using System;
using System.IO;
using System.Text.RegularExpressions;

public class Script {
	public static void Main (string[] args) {
		string text = @"qwerty-trewq-qwerty-werty-trewq-erty-trewq-rty";
		Regex rx = new Regex(@"-(?!tr)\w*ty"); // начинаются на "-" (минус), заканчиваются на "ty", не содержат "tr"...
     // Find matches.
     MatchCollection matches = rx.Matches(text);

     // Report the number of matches found.
     Console.WriteLine("{0} matches found.", matches.Count);

     // Report on each match.
     foreach (Match match in matches)
     {
         string word = match.Groups[0].Value;
         int index = match.Index;
         Console.WriteLine("{0} repeated at position {1}", word, index);   
     }
	}
}
//////////////// Output:

4 matches found.
-qwerty repeated at position 12
-werty repeated at position 19
-erty repeated at position 31
-rty repeated at position 42

...
Regex rx = new Regex(@"-(?!tr)\w*ty");
...