const string pattern = @"(?<duplicateWord>\w+)\s\k<duplicateWord>\W(?<nextWord>\w+)";
const RegexOptions options = RegexOptions.IgnoreCase;
var rxTree = typeof(Regex).Assembly.GetType("System.Text.RegularExpressions.RegexParser")
  .GetMethod("Parse", BindingFlags.Static | BindingFlags.NonPublic)
  .Invoke(null, new object[] { pattern, options });
var result = (Hashtable)rxTree.GetType()
  .GetField("_capnames", BindingFlags.NonPublic | BindingFlags.Instance)
  .GetValue(rxTree);
foreach (DictionaryEntry de in result)
  Console.WriteLine("{0} {1}", de.Key, de.Value);