using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
string pattern = @"X\s*([^\sX](?:[^X]*[^\sX])?)";
string input = @"X
aaaa
X
bbbb
X
cccc
XX";

foreach (Match m in Regex.Matches(input, pattern))
{
    Console.WriteLine(m.Groups[1].Value);
}
	}
}