IronRuby.Builtins.RegexpTransformer.ParseCharacterGroup C# (CSharp) Method

        private CharacterSet/*!*/ ParseCharacterGroup(bool nested) {
            Debug.Assert(_rubyPattern[_index - 1] == '[');

            bool positive = !Read('^');

            // [:alnum:]
            // [^:alnum:]
            if (nested) {
                var posixClass = ParsePosixCharacterClass(positive);
                if (posixClass != null) {
                    return posixClass;
                }
            }

            var result = ParseCharacterGroupIntersections();
            if (!positive) {
                result = result.Complement();
            }
            Debug.Assert(Peek() == -1 || Peek() == ']');
            Read(']');
            return result;
        }