using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        List<String> list = new List<String>() { "a 1", "a 20", "a 2", "a 12", "a 25", "b 1", "b 5"};

        var ordered = list.OrderBy(x => PadNumbers(x));

        foreach (var items in ordered)

        {

            Console.WriteLine(items);

        }

    }

    public static string PadNumbers(string input)

    {

        return Regex.Replace(input, "[0-9]+", match => match.Value.PadLeft(2, '0'));

    }

}