using System;

using System.Text.RegularExpressions;

class Program

{

    static void Main()

    {

        string sentence = "School had 40 Rooms, 500 Boys, 500 Girls and 250 Teachers";

        string[] digits = Regex.Split(sentence, @"\D+");

        foreach (string value in digits)

        {

            int number;

            if (int.TryParse(value, out number))

            {

                Console.Write(value);

            }

            Console.ReadLine();

        }

    }

}

​