using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var numbers = new List<long>{2001234567, 200123456789, 2001, 567};

        var regex = new Regex("\b\\d{10}\b", RegexOptions.IgnoreCase);

        foreach (var number in numbers)

        {

            var result = regex.IsMatch(number.ToString());

            Console.WriteLine($"Number: {number}, IsValid: {result}");

        }

    }

}