using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var lst = new List<string> {"70707_70708_70709_display1.jpg",
			"70707_Front010.jpg",
			"626-this files is tagged.jpg",
			"1000x1000_webbanner2.jpg",
			"2000 years ago_files.jpg",
			"626gamingassets_styleguide.jpg" };
	        foreach (var s in lst) 
	        {
	        	var mcoll = Regex.Matches(s, @"^(?<v>\d{3,})(?:_(?<v>\d{3,}))*_")
	        		.Cast<Match>()
	        		.SelectMany(m => m.Groups["v"].Captures.Cast<Capture>().Select(c => c.Value))
	        		.ToList();
	        	if (mcoll.Count > 0)
	        		Console.WriteLine("Results: {0}", string.Join(", ", mcoll));
	        	else
	        		Console.WriteLine("No match: '{0}'", s);
	        }
	}
}