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

public class Test
{
	public static void Main()
	{
		var rx = @"(WordA\s)?(?:WordB\s)?WordC";
		var strings = new List<String> {"WordC", "WordB WordC", "WordA WordC", "WordA WordB WordC"};
		foreach (var s in strings)
		{
			var m = Regex.Match(s, rx);
			Console.WriteLine("{0}: {1}", s, (m.Groups[1].Success ? "NO MATCH" : m.Value));
		}
	}
}