/// <summary>
        /// This is the only function that should be called from outside.
        /// It takes a RegexTree and creates a corresponding RegexCode.
        /// </summary>
        internal static RegexCode Write(RegexTree t)
        {
            RegexWriter w      = new RegexWriter();
            RegexCode   retval = w.RegexCodeFromRegexTree(t);

#if DEBUG
            if (t.Debug)
            {
                t.Dump();
                retval.Dump();
            }
#endif
            return(retval);
        }
        private Regex(string pattern, RegexOptions options, TimeSpan matchTimeout, bool useCache)
        {
            RegexTree       tree;
            CachedCodeEntry cached     = null;
            string          cultureKey = null;

            if (pattern == null)
            {
                throw new ArgumentNullException(nameof(pattern));
            }
            if (options < RegexOptions.None || (((int)options) >> MaxOptionShift) != 0)
            {
                throw new ArgumentOutOfRangeException(nameof(options));
            }
            if ((options & RegexOptions.ECMAScript) != 0 &&
                (options & ~(RegexOptions.ECMAScript |
                             RegexOptions.IgnoreCase |
                             RegexOptions.Multiline |
                             RegexOptions.Compiled |
                             RegexOptions.CultureInvariant
#if DEBUG
                             | RegexOptions.Debug
#endif
                             )) != 0)
            {
                throw new ArgumentOutOfRangeException(nameof(options));
            }

            ValidateMatchTimeout(matchTimeout);

            // Try to look up this regex in the cache.  We do this regardless of whether useCache is true since there's
            // really no reason not to.
            if ((options & RegexOptions.CultureInvariant) != 0)
            {
                cultureKey = CultureInfo.InvariantCulture.ToString(); // "English (United States)"
            }
            else
            {
                cultureKey = CultureInfo.CurrentCulture.ToString();
            }

            var key = new CachedCodeEntryKey(options, cultureKey, pattern);
            cached = LookupCachedAndUpdate(key);

            this.pattern = pattern;
            roptions     = options;

            internalMatchTimeout = matchTimeout;

            if (cached == null)
            {
                // Parse the input
                tree = RegexParser.Parse(pattern, roptions);

                // Extract the relevant information
                capnames = tree._capnames;
                capslist = tree._capslist;
                _code    = RegexWriter.Write(tree);
                caps     = _code._caps;
                capsize  = _code._capsize;

                InitializeReferences();

                tree = null;
                if (useCache)
                {
                    cached = CacheCode(key);
                }
            }
            else
            {
                caps             = cached._caps;
                capnames         = cached._capnames;
                capslist         = cached._capslist;
                capsize          = cached._capsize;
                _code            = cached._code;
                _runnerref       = cached._runnerref;
                _replref         = cached._replref;
                _refsInitialized = true;
            }

            // if the compile option is set, then compile the code if it's not already
            if (UseOptionC() && factory == null)
            {
                factory = RegexCompiler.Compile(_code, roptions);

                if (useCache && cached != null)
                {
                    cached.AddCompiled(factory);
                }
                _code = null;
            }
        }