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

public class Folders
{
    public static IEnumerable<string> FolderNames(string xml, char startingLetter)
    {
        List<string> result = new List<string>();

        char[] separator = {'"'};

        string[] wordsArray = xml.Split(separator, StringSplitOptions.RemoveEmptyEntries);

        for (int i = 0; i < wordsArray.Length-2; i++)
        {
            if (wordsArray[i].Contains("folder name=") && wordsArray[i+1].StartsWith($"{startingLetter}"))
            {
                result.Add(wordsArray[i + 1]);
            }
        }


        return result;
    }

    public static void Main(string[] args)
    {
        string xml =
            "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
            "<folder name=\"c\">" +
                "<folder name=\"program files\">" +
                    "<folder name=\"uninstall information\" />" +
                "</folder>" +
                "<folder name=\"users\" />" +
            "</folder>";

        foreach (string name in Folders.FolderNames(xml, 'u'))
            Console.WriteLine(name);

        Console.ReadLine();
    }
}