About Regular Expressions

As the output strings are not simple sub-strings of the original string, you need several steps to break this down using regular expressions.

Try the following which I've tried to generalize so that it can deal with more than 2 strings in the comma separated list:

using System;
using System.Text.RegularExpressions;

class Test
{
   static void Main()
   {
      string str = "ALLRAD + !(812_ENGLAND, US)";
      string regExp = @"^(\w+\s*\+\s*(!)?)\((\w+(,\s*\w+)*)\)$";      
      Match m = Regex.Match(str, regExp);
      string[] split = Regex.Split(m.Groups[3].Value, @",\s*");
      string[] output = new string[split.Length];

      for(int i = 0; i < split.Length; i++)
      {
         output[i] = String.Format("({0}({1}))", m.Groups[1], split[i]);
         Console.WriteLine(output[i]);
      }

      Console.ReadKey(); 
   }
}

The output is:

(ALLRAD + !(812_ENGLAND))
(ALLRAD + !(US))

If the original string had been:

 "ALLRAD + !(812_ENGLAND, US,FRANCE,PORTUGAL)"

the output would have been:

(ALLRAD + !(812_ENGLAND))
(ALLRAD + !(US))
(ALLRAD + !(FRANCE))
(ALLRAD + !(PORTUGAL))