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[] {"REGISTER_OPTION_P2", "REGISTER_OPTION_P27", "REGISTER_OPTION_P254","REGISTER_OPTION_NOFW", "POWER_OPTION_P45JW"};
		var pattern = @"_P[0-9]*$";
    	var result = lst.Where(x => Regex.IsMatch(x, pattern, RegexOptions.RightToLeft));
        foreach (var s in result)
        	Console.WriteLine(s);
	}
}