C#

using System;
using System.Linq;

namespace ConsoleApplication1
{
   class Program
   {
      static void Main(string[] args)
      {
         Tester();
         Console.ReadLine();
      }
      public static void Tester()
      {
         //Bogus variables to emulate code
         int[] zeroFrequencyLetterList = { 1, 2, 3, 4, 0, 5, 10, 9, 8, 7, 6, 5, 10, 2, 9, 7, 0, 6 };
         int decrypted = 0;
         char[] encryptionKey = { 'a', 'b', ' ', 'c', 'd', 'e', ' ', ' ', 'f', ' ', 'g' };

         Random r = new Random();
         for (int i = 0; i < zeroFrequencyLetterList.Count() && decrypted < 25; i++)
         {
            int randomed;
            char aLetter;
            do
            {
               randomed = r.Next(0, zeroFrequencyLetterList.Count());
               aLetter = encryptionKey[zeroFrequencyLetterList[randomed]];
               Console.WriteLine("{0,3}: {1,3} -> {2}", i, randomed, aLetter ); 
            } while (aLetter == ' ');

            //    encryptionKey[zeroFrequencyLetterList[i]] = abcLetters[randomed];
            //    abcLetters.RemoveAt(randomed);
            //    zeroFrequencyLetterList.RemoveAt(i);
            decrypted++;
         }
      }
   }
}

OriginalGriff wrote:

Your first problem is the spaces between the digits

Regex rx = new Regex("((?:\\+27|27)|0)(72|82|73|83|74|84)(\\d{7})");