/// <summary>
        /// Attempts to parse a markdown link e.g. "[](http://www.reddit.com)".
        /// </summary>
        /// <param name="markdown"> The markdown text. </param>
        /// <param name="start"> The location to start parsing. </param>
        /// <param name="maxEnd"> The location to stop parsing. </param>
        /// <returns> A parsed markdown link, or <c>null</c> if this is not a markdown link. </returns>
        internal static Helpers.Common.InlineParseResult Parse(string markdown, int start, int maxEnd)
        {
            // Expect a '[' character.
            if (start == maxEnd || markdown[start] != '[')
            {
                return(null);
            }

            // Find the ']' character, keeping in mind that [test [0-9]](http://www.test.com) is allowed.
            int linkTextOpen = start + 1;
            int pos          = linkTextOpen;
            int linkTextClose;
            int openSquareBracketCount = 0;

            while (true)
            {
                linkTextClose = markdown.IndexOfAny(new char[] { '[', ']' }, pos, maxEnd - pos);
                if (linkTextClose == -1)
                {
                    return(null);
                }

                if (markdown[linkTextClose] == '[')
                {
                    openSquareBracketCount++;
                }
                else if (openSquareBracketCount > 0)
                {
                    openSquareBracketCount--;
                }
                else
                {
                    break;
                }

                pos = linkTextClose + 1;
            }

            // Skip whitespace.
            pos = linkTextClose + 1;
            while (pos < maxEnd && Helpers.Common.IsWhiteSpace(markdown[pos]))
            {
                pos++;
            }

            if (pos == maxEnd)
            {
                return(null);
            }

            // Expect the '(' character or the '[' character.
            int linkOpen = pos;

            if (markdown[pos] == '(')
            {
                // Skip whitespace.
                linkOpen++;
                while (linkOpen < maxEnd && Helpers.Common.IsWhiteSpace(markdown[linkOpen]))
                {
                    linkOpen++;
                }

                // Find the ')' character.
                pos = linkOpen;
                int linkClose = -1;
                while (pos < maxEnd)
                {
                    linkClose = Helpers.Common.IndexOf(markdown, ')', pos, maxEnd);
                    if (linkClose == -1)
                    {
                        return(null);
                    }

                    if (markdown[linkClose - 1] != '\\')
                    {
                        break;
                    }

                    pos = linkClose + 1;
                }

                if (pos >= maxEnd)
                {
                    return(null);
                }

                int end = linkClose + 1;

                // Skip whitespace backwards.
                while (linkClose > linkOpen && Helpers.Common.IsWhiteSpace(markdown[linkClose - 1]))
                {
                    linkClose--;
                }

                // If there is no text whatsoever, then this is not a valid link.
                if (linkOpen == linkClose)
                {
                    return(null);
                }

                // Check if there is tooltip text.
                string url;
                string tooltip = null;
                bool   lastUrlCharIsDoubleQuote = markdown[linkClose - 1] == '"';
                int    tooltipStart             = Helpers.Common.IndexOf(markdown, " \"", linkOpen, linkClose - 1);
                if (tooltipStart == linkOpen)
                {
                    return(null);
                }

                if (lastUrlCharIsDoubleQuote && tooltipStart != -1)
                {
                    // Extract the URL (resolving any escape sequences).
                    url     = TextRunInline.ResolveEscapeSequences(markdown, linkOpen, tooltipStart).TrimEnd(' ', '\t', '\r', '\n');
                    tooltip = markdown.Substring(tooltipStart + 2, (linkClose - 1) - (tooltipStart + 2));
                }
                else
                {
                    // Extract the URL (resolving any escape sequences).
                    url = TextRunInline.ResolveEscapeSequences(markdown, linkOpen, linkClose);
                }

                // Check the URL is okay.
                if (!IsUrlValid(url))
                {
                    return(null);
                }

                // We found a regular stand-alone link.
                var result = new MarkdownLinkInline();
                result.Inlines = Helpers.Common.ParseInlineChildren(markdown, linkTextOpen, linkTextClose, ignoreLinks: true);
                result.Url     = url.Replace(" ", "%20");
                result.Tooltip = tooltip;
                return(new Helpers.Common.InlineParseResult(result, start, end));
            }
            else if (markdown[pos] == '[')
            {
                // Find the ']' character.
                int linkClose = Helpers.Common.IndexOf(markdown, ']', pos + 1, maxEnd);
                if (linkClose == -1)
                {
                    return(null);
                }

                // We found a reference-style link.
                var result = new MarkdownLinkInline();
                result.Inlines     = Helpers.Common.ParseInlineChildren(markdown, linkTextOpen, linkTextClose, ignoreLinks: true);
                result.ReferenceId = markdown.Substring(linkOpen + 1, linkClose - (linkOpen + 1));
                if (result.ReferenceId == string.Empty)
                {
                    result.ReferenceId = markdown.Substring(linkTextOpen, linkTextClose - linkTextOpen);
                }

                return(new Helpers.Common.InlineParseResult(result, start, linkClose + 1));
            }

            return(null);
        }
        /// <summary>
        /// Attempts to parse a reference e.g. "[example]: http://www.reddit.com 'title'".
        /// </summary>
        /// <param name="markdown"> The markdown text. </param>
        /// <param name="start"> The location to start parsing. </param>
        /// <param name="end"> The location to stop parsing. </param>
        /// <returns> A parsed markdown link, or <c>null</c> if this is not a markdown link. </returns>
        internal static LinkReferenceBlock Parse(string markdown, int start, int end)
        {
            // Expect a '[' character.
            if (start >= end || markdown[start] != '[')
            {
                return(null);
            }

            // Find the ']' character
            int pos = start + 1;

            while (pos < end)
            {
                if (markdown[pos] == ']')
                {
                    break;
                }

                pos++;
            }

            if (pos == end)
            {
                return(null);
            }

            // Extract the ID.
            string id = markdown.Substring(start + 1, pos - (start + 1));

            // Expect the ':' character.
            pos++;
            if (pos == end || markdown[pos] != ':')
            {
                return(null);
            }

            // Skip whitespace
            pos++;
            while (pos < end && Helpers.Common.IsWhiteSpace(markdown[pos]))
            {
                pos++;
            }

            if (pos == end)
            {
                return(null);
            }

            // Extract the URL.
            int urlStart = pos;

            while (pos < end && !Helpers.Common.IsWhiteSpace(markdown[pos]))
            {
                pos++;
            }

            string url = TextRunInline.ResolveEscapeSequences(markdown, urlStart, pos);

            // Ignore leading '<' and trailing '>'.
            url = url.TrimStart('<').TrimEnd('>');

            // Skip whitespace.
            pos++;
            while (pos < end && Helpers.Common.IsWhiteSpace(markdown[pos]))
            {
                pos++;
            }

            string tooltip = null;

            if (pos < end)
            {
                // Extract the tooltip.
                char tooltipEndChar;
                switch (markdown[pos])
                {
                case '(':
                    tooltipEndChar = ')';
                    break;

                case '"':
                case '\'':
                    tooltipEndChar = markdown[pos];
                    break;

                default:
                    return(null);
                }

                pos++;
                int tooltipStart = pos;
                while (pos < end && markdown[pos] != tooltipEndChar)
                {
                    pos++;
                }

                if (pos == end)
                {
                    return(null);    // No end character.
                }

                tooltip = markdown.Substring(tooltipStart, pos - tooltipStart);

                // Check there isn't any trailing text.
                pos++;
                while (pos < end && Helpers.Common.IsWhiteSpace(markdown[pos]))
                {
                    pos++;
                }

                if (pos < end)
                {
                    return(null);
                }
            }

            // We found something!
            var result = new LinkReferenceBlock();

            result.Id      = id;
            result.Url     = url;
            result.Tooltip = tooltip;
            return(result);
        }