NValidate

protected int getPlus4(string zipCode)
{
   const string ZipPlus4Pattern = "\\d[5]-\\d[4]";
   const string ZipPlus4Error = "Not a valid ZIP+4 Code.";

   if (zipCode == null)
      throw new ArgumentNullException("zipCode");
   if (zipCode == string.Empty)
      throw new ArgumentException("Empty
          strings not permitted.");
   if (!((new Regex(ZipPlus4Pattern)).IsMatch(zipCode)))
      throw new ArgumentNullException(ZipPlus4Error, "zipCode");

   return int.Parse(zipCode.Substring(6, 4));
}

protected int getPlus4(string zipCode)
{
   const string ZipPlus4Pattern = "\\d[5]-\\d[4]";
   const string ZipPlus4Error = "Not a valid ZIP+4 Code.";

   Demand.That(zipCode, "zipCode").IsNotNullOrEmpty().Matches(ZipPlus4Pattern, ZipPlus4Error);

   return int.Parse(zipCode.Substring(6, 4));
}