select list items by using print page like string

GenericExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

// TODO: check out of page ranges.
public static class GenericExtensions
    {
         public static List<T> PrintPagesFilter<T>(this List<T> list, string pattern)
        {
            List<T> NewList = new List<T>();

            foreach (string segment in pattern.Split(','))
            {
                if (segment.Contains('-'))
                {
                    string[] limits = segment.Split('-');
                    int firstpage = Convert.ToInt32(limits[0].Trim());
                    int lastpage = Convert.ToInt32(limits[1].Trim());
                    int startindex = firstpage - 1;
                    int count = lastpage - startindex;

                    NewList.AddRange(list.GetRange(startindex, count));
                }
                else
                {
                    NewList.Add(list[Convert.ToInt32(segment.Trim()) - 1]);
                }
            }

            return NewList;
        }

        public static List<List<T>> Split<T>(this List<T> source, int junkSize)
        {
            return source
                .Select((x, i) => new { Index = i, Value = x })
                .GroupBy(x => x.Index / junkSize)
                .Select(x => x.Select(v => v.Value).ToList())
                .ToList();
        }

        public static List<List<T>> SplitColumn<T>(this List<T> source, int ColumnSize)
        {

            return source
                .Select((x, i) => new { Index = i, Value = x })
                .GroupBy(x => x.Index % (source.Count() / ColumnSize))
                .Select(x => x.Select(v => v.Value).ToList())
                .ToList();
        }
    }

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> Pages = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 }.ToList();
            
            List<int> PrintPages = Pages.PrintPagesFilter("1-5,10,11,15-20");

            Console.WriteLine(String.Join(",", PrintPages.Select(i => i.ToString()).ToArray()));
            // Output : 1,2,3,4,5,10,11,15,16,17,18,19,20
            Console.ReadKey();
        }
    }
}