using System;
using System.Linq;
using System.Text.RegularExpressions;

namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var implicitParenRegexes = @"
(a)(?(?=.) )(b) (c)
(a)(?(?!z) )(b) (c)
(a)(?(?<=.) )(b) (c)
(a)(?(?<! ) )(b) (c)
(a)(?(?: ) )(b) (c)
(a)(?(?i:.) )(b) (c)
(a)(?(?>.) )(b) (c)
(a)(?(?(1).) )(b) (c)
((?<n>a))(?(?(n).) )(b)(c)
(a)(?(?(?:.).) )(b) (c)
(a)(?(?<n>.) )(b) (c)
(a)(?(?'n'.) )(b) (c)
(a)(?(?'-n' .) )(b) (c)
(?<a>a)(?(?<a-n>.) )(b) (c)
(a)(?(?# comment) )(b) (c)
(a)(?(?i) )(b) (c)
(a)(?(?(.).) )(b) (c)
".Split(new char[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
            var explicitParenRegexes = @"
(a)(?((?=.)) )(b) (c)
(a)(?((?!z)) )(b) (c)
(a)(?((?<=.)) )(b) (c)
(a)(?((?<! )) )(b) (c)
(a)(?((?: )) )(b) (c)
(a)(?((?i:.)) )(b) (c)
(a)(?((?>.)) )(b) (c)
(a)(?((?(1).)) )(b) (c)
((?<n>a))(?((?(n).)) )(b)(c)
(a)(?((?(?:.).)) )(b) (c)
(a)(?((?<n>.)) )(b) (c)
(a)(?((?'n'.)) )(b) (c)
(a)(?((?'-n' .)) )(b) (c)
(?<a>a)(?((?<a-n>.)) )(b) (c)
(a)(?((?# comment)) )(b) (c)
(a)(?((?i)) )(b) (c)
(a)(?((?(.).)) )(b) (c)
".Split(new char[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);

            Console.WriteLine("IMPLICIT PARENTHESIZED CONDITIONS:");
            foreach (var pattern in implicitParenRegexes)
                CheckPattern(pattern);

            Console.WriteLine("\nEXPLICIT PARENTHESIZED CONDITIONS:");
            foreach (var pattern in explicitParenRegexes)
                CheckPattern(pattern);

            Console.ReadKey();
        }

        const string input = "a b c";
        static void CheckPattern(string pattern)
        {
            Console.Write("Regex: '{0}'; groups: ", pattern);

            // Comment this block to get the exception
            if (pattern.Contains("(?(?i)"))
            {
                Console.WriteLine("Skipped to allow it run in ideone");
                return;
            }

            try
            {
                int index = 1;
                Console.WriteLine(string.Join("; ", 
                    Regex.Match(input, pattern).Groups.Cast<Group>().Skip(1).Take(3)
                        .Select(g => string.Format("${0}: '{1}'", index++, g.Value))));
            }
            catch (Exception exception)
            {
                Console.WriteLine("ERROR: {0}: {1}", exception.GetType().Name, exception.Message);
            }
        }
	}
}