using System;
using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        string pattern = @"(\b|\s)(-{3})(\b|\s)";
        string input = @"This- -is - a test-sentence. -Test- --- One-Two--Three---Four----.";
        
        foreach (Match m in Regex.Matches(input, pattern))
        {
            Console.WriteLine("'{0}' found at index {1}.", m.Groups[2].Value, m.Index);
        }
    }
}