using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var Text = "substring1 is the substring2 document that was processed electronically";

        var words = Text.Split(" ").OrderBy(x => x).Distinct().ToList();

        var patterns = new List<string> {  "substring1", "substring2", "substring3", "substring4", "substring5" };

​

        List<string> matches = new();

        int patternIdx = 0, wordsIdx = 0;

        while(patternIdx < patterns.Count && wordsIdx < words.Count)

        {

            int comparision = string.Compare(patterns[patternIdx], words[wordsIdx]);

            switch(comparision)

            {

                case > 0: wordsIdx++; break;

                case < 0: patternIdx++; break;

                default: 

                {

                    matches.Add(patterns[patternIdx]); 

                    wordsIdx++;

                    patternIdx++;

                    break;

                }

            }

        }

        matches.Dump();

    }

}