filter with generator

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

namespace Filter
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 3)
            {
                Console.WriteLine("Usage: filter input output regex");
                return;
            }
            string input = args[0];
            string output = args[1];
            string regex = args[2];

            if (!File.Exists(input))
            {
                Console.WriteLine("Could not find input file.");
                return;
            }

            using (FileStream read = File.OpenRead(input))
            {
                using (TextReader reader = new StreamReader(read))
                {
                    File.WriteAllLines(output, GetLines(reader, regex));
                }
            }
        }

        private static IEnumerable<string> GetLines(TextReader reader, string regex)
        {
            Regex x = new Regex(regex);
            while (true)
            {
                string line = reader.ReadLine();
                if (line != null)
                {
                    if (x.IsMatch(line))
                    {
                        yield return line;
                    }
                }
                else
                {
                    break;
                }
            }
        }
    }
}