using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var str = "   Invisible Pty. Ltd.     1 Nowhere St.  Sydney  2000  AUSTRALIA "; //--> should be ["   ", "Invisible Pty. Ltd.", "     ", "1 Nowhere St.", "  ", "Sydney", "  ", "2000", "  ", "AUSTRALIA "]

        //str = " A teddy bear   A red firetruck ";    //--> should be [" A teddy bear", "   ", "A red firetruck "]

        //str = "  A teddy bear   A red firetruck   "; //--> should be ["  ", "A teddy bear", "   ", "A red firetruck,"   "]

        //tokenize the input delimited by 2 or more whitespaces

        var tokens = str.TokenizeUsingRegex(@"\s{2,}");

        foreach(var token in tokens)

        {

            Console.WriteLine("'{0}' - {1}", token, token.Length);

        }

    }

}

​

public static class StringExtension

{

    public static List<string> TokenizeUsingRegex(this string input, string separatorRegexPattern, bool includeSeparatorsAsToken = true)

    {

        var tokens = Regex.Split(input, separatorRegexPattern).Where(t => !string.IsNullOrWhiteSpace(t)).ToList();

        if (!includeSeparatorsAsToken)

            return tokens;

        //Reinstate the removed separators      

        var newTokens = new List<string>();

        var startIndex  = 0;

        for(int i = 0, l = tokens.Count(); i < l; i++) 

        {

            var token = tokens[i];          

            var endIndex = input.IndexOf(token);

            if (startIndex < endIndex) {

                //Add back the separator as a new token

                newTokens.Add(input.Substring(startIndex, endIndex - startIndex));

            }

            //Then add the token afterward

            newTokens.Add(token);

            startIndex = endIndex + token.Length;           

        }

        //Add last separator if any

        if (startIndex < input.Length) {            

            newTokens.Add(input.Substring(startIndex));

        }

        return newTokens;

    }   

}