using System;

using System.Linq;

public class Program

{

    public static void Main()

    {       

        const int count = 1000000;

        var str = "123456789f";

        var stopwatch = System.Diagnostics.Stopwatch.StartNew();

        for (var i = 0; i < count; i++)

        {           

            var isMatch = System.Text.RegularExpressions.Regex.IsMatch(str, @"^\d{10}$");

        }

        Console.WriteLine("Regex: " + stopwatch.ElapsedMilliseconds);

        stopwatch.Restart();

        for (var i = 0; i < count; i++)

        {

            var isMatch =

                str.Length == 10

                && str.All(c => '0' <= c && c <= '9');

        }       

        Console.WriteLine("LINQ: " + stopwatch.ElapsedMilliseconds);

    }

}