using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string[] words = { "testing", "world", "ראשוני", "new", "hello", "test" };

​

        List<string> newText = new List<string>() {         

            "This line dosent match",

            "date1",

            "",

            "This line does match with the word: hello",

            "date2",

            "",

            "This line dosent match either",

            "date1",

            "",

            "This line does match with the word: world",

            "date4",

            "",

        };

​

        int i = 0;

​

        while (i < newText.Count)

        {            

            // Get an array of words

            string[] lineWords = newText[i].Split(' ');

​

            if (lineWords.Intersect(words).Count() == 0)

            {

                // This line has no matching words. Remove 3 lines.

                for (int n = 0; n < 3; n++)

                    newText.RemoveAt(i);

            }

            else

            {

                // This line has matching words. Move forward 3 lines.

                i += 3;

            }

        }

​

        foreach (string line in newText)

            Console.WriteLine(line);

    }

}