src/Markdown.cs

/*
 * MarkdownSharp
 * -------------
 * a C# Markdown processor
 * 
 * Markdown is a text-to-HTML conversion tool for web writers
 * Copyright (c) 2004 John Gruber
 * http://daringfireball.net/projects/markdown/
 * 
 * Markdown.NET
 * Copyright (c) 2004-2009 Milan Negovan
 * http://www.aspnetresources.com
 * http://aspnetresources.com/blog/markdown_announced.aspx
 * 
 * MarkdownSharp
 * Copyright (c) 2009-2010 Jeff Atwood
 * http://stackoverflow.com
 * http://www.codinghorror.com/blog/
 * http://code.google.com/p/markdownsharp/
 * 
 * History: Milan ported the Markdown processor to C#. He granted license to me so I can open source it
 * and let the community contribute to and improve MarkdownSharp.
 * 
 */

#region Copyright and license

/*

Copyright (c) 2009 - 2010 Jeff Atwood

http://www.opensource.org/licenses/mit-license.php
  
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

Copyright (c) 2003-2004 John Gruber
<http://daringfireball.net/>   
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

* Redistributions of source code must retain the above copyright notice,
  this list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright
  notice, this list of conditions and the following disclaimer in the
  documentation and/or other materials provided with the distribution.

* Neither the name "Markdown" nor the names of its contributors may
  be used to endorse or promote products derived from this software
  without specific prior written permission.

This software is provided by the copyright holders and contributors "as
is" and any express or implied warranties, including, but not limited
to, the implied warranties of merchantability and fitness for a
particular purpose are disclaimed. In no event shall the copyright owner
or contributors be liable for any direct, indirect, incidental, special,
exemplary, or consequential damages (including, but not limited to,
procurement of substitute goods or services; loss of use, data, or
profits; or business interruption) however caused and on any theory of
liability, whether in contract, strict liability, or tort (including
negligence or otherwise) arising in any way out of the use of this
software, even if advised of the possibility of such damage.
*/

#endregion

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Text;
using System.Text.RegularExpressions;

namespace MarkdownSharp
{

    public class MarkdownOptions
    {
        public bool AutoHyperlink { get; set; }
        public bool AutoNewlines { get; set; }
        public string EmptyElementSuffix { get; set; }
        public bool EncodeProblemUrlCharacters { get; set; }
        public bool LinkEmails { get; set; }
        public bool StrictBoldItalic { get; set; }
    }


    public class Markdown
    {
        private const string _version = "1.13";

        #region Constructors and Options

        public Markdown() : this(false)
        {
        }

        public Markdown(bool loadOptionsFromConfigFile)
        {
            if (!loadOptionsFromConfigFile) return;

            var settings = ConfigurationManager.AppSettings;
            foreach (string key in settings.Keys)
            {
                switch (key)
                {
                    case "Markdown.AutoHyperlink":
                        _autoHyperlink = Convert.ToBoolean(settings[key]);
                        break;
                    case "Markdown.AutoNewlines":
                        _autoNewlines = Convert.ToBoolean(settings[key]);
                        break;
                    case "Markdown.EmptyElementSuffix":
                        _emptyElementSuffix = settings[key];
                        break;
                    case "Markdown.EncodeProblemUrlCharacters":
                        _encodeProblemUrlCharacters = Convert.ToBoolean(settings[key]);
                        break;
                    case "Markdown.LinkEmails":
                        _linkEmails = Convert.ToBoolean(settings[key]);
                        break;
                    case "Markdown.StrictBoldItalic":
                        _strictBoldItalic = Convert.ToBoolean(settings[key]);
                        break;
                }
            }
        }

        public Markdown(MarkdownOptions options)
        {
            _autoHyperlink = options.AutoHyperlink;
            _autoNewlines = options.AutoNewlines;
            _emptyElementSuffix = options.EmptyElementSuffix;
            _encodeProblemUrlCharacters = options.EncodeProblemUrlCharacters;
            _linkEmails = options.LinkEmails;
            _strictBoldItalic = options.StrictBoldItalic;
        }


        public string EmptyElementSuffix
        {
            get { return _emptyElementSuffix; }
            set { _emptyElementSuffix = value; }
        }
        private string _emptyElementSuffix = " />";

        public bool LinkEmails
        {
            get { return _linkEmails; }
            set { _linkEmails = value; }
        }
        private bool _linkEmails = true;

        public bool StrictBoldItalic
        {
            get { return _strictBoldItalic; }
            set { _strictBoldItalic = value; }
        }
        private bool _strictBoldItalic = false;

        public bool AutoNewLines
        {
            get { return _autoNewlines; }
            set { _autoNewlines = value; }
        }
        private bool _autoNewlines = false;

        public bool AutoHyperlink
        {
            get { return _autoHyperlink; }
            set { _autoHyperlink = value; }
        }
        private bool _autoHyperlink = false;

        public bool EncodeProblemUrlCharacters
        {
            get { return _encodeProblemUrlCharacters; }
            set { _encodeProblemUrlCharacters = value; }
        }
        private bool _encodeProblemUrlCharacters = false;

        #endregion

        private enum TokenType { Text, Tag }

        private struct Token
        {
            public Token(TokenType type, string value)
            {
                this.Type = type;
                this.Value = value;
            }
            public TokenType Type;
            public string Value;
        }

        private const int _nestDepth = 6;

        private const int _tabWidth = 4;

        private const string _markerUL = @"[*+-]";
        private const string _markerOL = @"\d+[.]";

        private static readonly Dictionary<string, string> _escapeTable;
        private static readonly Dictionary<string, string> _invertedEscapeTable;
        private static readonly Dictionary<string, string> _backslashEscapeTable;        

        private readonly Dictionary<string, string> _urls = new Dictionary<string, string>();
        private readonly Dictionary<string, string> _titles = new Dictionary<string, string>();
        private readonly Dictionary<string, string> _htmlBlocks = new Dictionary<string, string>();

        private int _listLevel;

        static Markdown()
        {
            // Table of hash values for escaped characters:
            _escapeTable = new Dictionary<string, string>();
            _invertedEscapeTable = new Dictionary<string, string>();
            // Table of hash value for backslash escaped characters:
            _backslashEscapeTable = new Dictionary<string, string>();

            string backslashPattern = "";

            foreach (char c in @"\`*_{}[]()>#+-.!")
            {
                string key = c.ToString();
                string hash = GetHashKey(key);
                _escapeTable.Add(key, hash);
                _invertedEscapeTable.Add(hash, key);
                _backslashEscapeTable.Add(@"\" + key, hash);
                backslashPattern += Regex.Escape(@"\" + key) + "|";
            }

            _backslashEscapes = new Regex(backslashPattern.Substring(0, backslashPattern.Length - 1), RegexOptions.Compiled);
        }

        public string Version
        {
            get { return _version; }
        }

        public string Transform(string text)
        {
            if (String.IsNullOrEmpty(text)) return "";

            Setup();

            text = Normalize(text);
           
            text = HashHTMLBlocks(text);
            text = StripLinkDefinitions(text);
            text = RunBlockGamut(text);
            text = Unescape(text);

            Cleanup();

            return text + "\n";
        }


        private string RunBlockGamut(string text)
        {
            text = DoHeaders(text);
            text = DoHorizontalRules(text);
            text = DoLists(text);
            text = DoCodeBlocks(text);
            text = DoBlockQuotes(text);

            // We already ran HashHTMLBlocks() before, in Markdown(), but that
            // was to escape raw HTML in the original Markdown source. This time,
            // we're escaping the markup we've just created, so that we don't wrap
            // <p> tags around block-level tags.
            text = HashHTMLBlocks(text);

            text = FormParagraphs(text);

            return text;
        }


        private string RunSpanGamut(string text)
        {
            text = DoCodeSpans(text);
            text = EscapeSpecialCharsWithinTagAttributes(text);
            text = EscapeBackslashes(text);

            // Images must come first, because ![foo][f] looks like an anchor.
            text = DoImages(text);
            text = DoAnchors(text);

            // Must come after DoAnchors(), because you can use < and >
            // delimiters in inline links like [this](<url>).
            text = DoAutoLinks(text);

            text = EncodeAmpsAndAngles(text);
            text = DoItalicsAndBold(text);
            text = DoHardBreaks(text);

            return text;
        }

        private static Regex _newlinesLeadingTrailing = new Regex(@"^\n+|\n+\z", RegexOptions.Compiled);
        private static Regex _newlinesMultiple = new Regex(@"\n{2,}", RegexOptions.Compiled);
        private static Regex _leadingWhitespace = new Regex(@"^[ ]*", RegexOptions.Compiled);

        private string FormParagraphs(string text)
        {
            // split on two or more newlines
            string[] grafs = _newlinesMultiple.Split(_newlinesLeadingTrailing.Replace(text, ""));
            
            for (int i = 0; i < grafs.Length; i++)
            {
                if (grafs[i].StartsWith("\x1A"))
                {
                    // unhashify HTML blocks
                    grafs[i] = _htmlBlocks[grafs[i]];
                }
                else
                {
                    // do span level processing inside the block, then wrap result in <p> tags
                    grafs[i] = _leadingWhitespace.Replace(RunSpanGamut(grafs[i]), "<p>") + "</p>";
                }
            }

            return string.Join("\n\n", grafs);
        }


        private void Setup()
        {
            // Clear the global hashes. If we don't clear these, you get conflicts
            // from other articles when generating a page which contains more than
            // one article (e.g. an index page that shows the N most recent
            // articles):
            _urls.Clear();
            _titles.Clear();
            _htmlBlocks.Clear();
            _listLevel = 0;
        }

        private void Cleanup()
        {
            Setup();
        }

        private static string _nestedBracketsPattern;

        private static string GetNestedBracketsPattern()
        {
            // in other words [this] and [this[also]] and [this[also[too]]]
            // up to _nestDepth
            if (_nestedBracketsPattern == null)
                _nestedBracketsPattern =
                    RepeatString(@"
                    (?>              # Atomic matching
                       [^\[\]]+      # Anything other than brackets
                     |
                       \[
                           ", _nestDepth) + RepeatString(
                    @" \]
                    )*"
                    , _nestDepth);
            return _nestedBracketsPattern;
        }

        private static string _nestedParensPattern;

        private static string GetNestedParensPattern()
        {
            // in other words (this) and (this(also)) and (this(also(too)))
            // up to _nestDepth
            if (_nestedParensPattern == null)
                _nestedParensPattern =
                    RepeatString(@"
                    (?>              # Atomic matching
                       [^()\s]+      # Anything other than parens or whitespace
                     |
                       \(
                           ", _nestDepth) + RepeatString(
                    @" \)
                    )*"
                    , _nestDepth);
            return _nestedParensPattern;
        }

        private static Regex _linkDef = new Regex(string.Format(@"
                        ^[ ]{{0,{0}}}\[(.+)\]:  # id = $1
                          [ ]*
                          \n?                   # maybe *one* newline
                          [ ]*
                        <?(\S+?)>?              # url = $2
                          [ ]*
                          \n?                   # maybe one newline
                          [ ]*
                        (?:
                            (?<=\s)             # lookbehind for whitespace
                            [""(]
                            (.+?)               # title = $3
                            ["")]
                            [ ]*
                        )?                      # title is optional
                        (?:\n+|\Z)", _tabWidth - 1), RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private string StripLinkDefinitions(string text)
        {
            return _linkDef.Replace(text, new MatchEvaluator(LinkEvaluator));
        }

        private string LinkEvaluator(Match match)
        {
            string linkID = match.Groups[1].Value.ToLowerInvariant();
            _urls[linkID] = EncodeAmpsAndAngles(match.Groups[2].Value);

            if (match.Groups[3] != null && match.Groups[3].Length > 0)
                _titles[linkID] = match.Groups[3].Value.Replace("\"", "&quot;");

            return "";
        }

        // compiling this monster regex results in worse performance. trust me.
        private static Regex _blocksHtml = new Regex(GetBlockPattern(), RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace);


        private static string GetBlockPattern()
        {

            // Hashify HTML blocks:
            // We only want to do this for block-level HTML tags, such as headers,
            // lists, and tables. That's because we still want to wrap <p>s around
            // "paragraphs" that are wrapped in non-block-level tags, such as anchors,
            // phrase emphasis, and spans. The list of tags we're looking for is
            // hard-coded:
            //
            // *  List "a" is made of tags which can be both inline or block-level.
            //    These will be treated block-level when the start tag is alone on 
            //    its line, otherwise they're not matched here and will be taken as 
            //    inline later.
            // *  List "b" is made of tags which are always block-level;
            //
            string blockTagsA = "ins|del";
            string blockTagsB = "p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|address|script|noscript|form|fieldset|iframe|math";

            // Regular expression for the content of a block tag.
            string attr = @"
            (?>                         # optional tag attributes
              \s                        # starts with whitespace
              (?>
                [^>""/]+                # text outside quotes
              |
                /+(?!>)                 # slash not followed by >
              |
                ""[^""]*""              # text inside double quotes (tolerate >)
              |
                '[^']*'                 # text inside single quotes (tolerate >)
              )*
            )?  
            ";

            string content = RepeatString(@"
                (?>
                  [^<]+                 # content without tag
                |
                  <\2                   # nested opening tag
                    " + attr + @"       # attributes
                  (?>
                      />
                  |
                      >", _nestDepth) +   // end of opening tag
                      ".*?" +             // last level nested tag content
            RepeatString(@"
                      </\2\s*>          # closing nested tag
                  )
                  |             
                  <(?!/\2\s*>           # other tags with a different name
                  )
                )*", _nestDepth);

            string content2 = content.Replace(@"\2", @"\3");

            // First, look for nested blocks, e.g.:
            //  <div>
            //      <div>
            //      tags for inner block must be indented.
            //      </div>
            //  </div>
            //
            // The outermost tags must start at the left margin for this to match, and
            // the inner nested divs must be indented.
            // We need to do this before the next, more liberal match, because the next
            // match will start at the first `<div>` and stop at the first `</div>`.
            string pattern = @"
            (?>
                  (?>
                    (?<=\n)     # Starting after a blank line
                    |           # or
                    \A\n?       # the beginning of the doc
                  )
                  (             # save in $1

                    # Match from `\n<tag>` to `</tag>\n`, handling nested tags 
                    # in between.
                      
                        [ ]{0,$less_than_tab}
                        <($block_tags_b_re)   # start tag = $2
                        $attr>                # attributes followed by > and \n
                        $content              # content, support nesting
                        </\2>                 # the matching end tag
                        [ ]*                  # trailing spaces
                        (?=\n+|\Z)            # followed by a newline or end of document

                  | # Special version for tags of group a.

                        [ ]{0,$less_than_tab}
                        <($block_tags_a_re)   # start tag = $3
                        $attr>[ ]*\n          # attributes followed by >
                        $content2             # content, support nesting
                        </\3>                 # the matching end tag
                        [ ]*                  # trailing spaces
                        (?=\n+|\Z)            # followed by a newline or end of document
                      
                  | # Special case just for <hr />. It was easier to make a special 
                    # case than to make the other regex more complicated.
                  
                        [ ]{0,$less_than_tab}
                        <(hr)                 # start tag = $2
                        $attr                 # attributes
                        /?>                   # the matching end tag
                        [ ]*
                        (?=\n{2,}|\Z)         # followed by a blank line or end of document
                  
                  | # Special case for standalone HTML comments:
                  
                      [ ]{0,$less_than_tab}
                      (?s:
                        <!-- .*? -->
                      )
                      [ ]*
                      (?=\n{2,}|\Z)            # followed by a blank line or end of document
                  
                  | # PHP and ASP-style processor instructions (<? and <%)
                  
                      [ ]{0,$less_than_tab}
                      (?s:
                        <([?%])                # $2
                        .*?
                        \2>
                      )
                      [ ]*
                      (?=\n{2,}|\Z)            # followed by a blank line or end of document
                      
                  )
            )";

            pattern = pattern.Replace("$less_than_tab", (_tabWidth - 1).ToString());
            pattern = pattern.Replace("$block_tags_b_re", blockTagsB);
            pattern = pattern.Replace("$block_tags_a_re", blockTagsA);
            pattern = pattern.Replace("$attr", attr);
            pattern = pattern.Replace("$content2", content2);
            pattern = pattern.Replace("$content", content);

            return pattern;
        }

        private string HashHTMLBlocks(string text)
        {
            return _blocksHtml.Replace(text, new MatchEvaluator(HtmlEvaluator));
        }

        private string HtmlEvaluator(Match match)
        {
            string text = match.Groups[1].Value;
            string key = GetHashKey(text);
            _htmlBlocks[key] = text;

            return string.Concat("\n\n", key, "\n\n");
        }

        private static string GetHashKey(string s)
        {
            return "\x1A" + Math.Abs(s.GetHashCode()).ToString() + "\x1A";
        }

        private static Regex _htmlTokens = new Regex(@"
            (<!(?:--.*?--\s*)+>)|        # match <!-- foo -->
            (<\?.*?\?>)|                 # match <?foo?> " +
            RepeatString(@" 
            (<[A-Za-z\/!$](?:[^<>]|", _nestDepth) + RepeatString(@")*>)", _nestDepth) +
                                       " # match <tag> and </tag>",
            RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.ExplicitCapture | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private List<Token> TokenizeHTML(string text)
        {
            int pos = 0;
            int tagStart = 0;
            var tokens = new List<Token>();

            // this regex is derived from the _tokenize() subroutine in Brad Choate's MTRegex plugin.
            // http://www.bradchoate.com/past/mtregex.php
            foreach (Match m in _htmlTokens.Matches(text))
            {
                tagStart = m.Index;

                if (pos < tagStart)
                    tokens.Add(new Token(TokenType.Text, text.Substring(pos, tagStart - pos)));

                tokens.Add(new Token(TokenType.Tag, m.Value));
                pos = tagStart + m.Length;
            }

            if (pos < text.Length)
                tokens.Add(new Token(TokenType.Text, text.Substring(pos, text.Length - pos)));

            return tokens;
        }


        private static Regex _anchorRef = new Regex(string.Format(@"
            (                               # wrap whole match in $1
                \[
                    ({0})                   # link text = $2
                \]

                [ ]?                        # one optional space
                (?:\n[ ]*)?                 # one optional newline followed by spaces

                \[
                    (.*?)                   # id = $3
                \]
            )", GetNestedBracketsPattern()), RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private static Regex _anchorInline = new Regex(string.Format(@"
                (                           # wrap whole match in $1
                    \[
                        ({0})               # link text = $2
                    \]
                    \(                      # literal paren
                        [ ]*
                        ({1})               # href = $3
                        [ ]*
                        (                   # $4
                        (['""])           # quote char = $5
                        (.*?)               # title = $6
                        \5                  # matching quote
                        [ ]*                # ignore any spaces between closing quote and )
                        )?                  # title is optional
                    \)
                )", GetNestedBracketsPattern(), GetNestedParensPattern()),
                  RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private static Regex _anchorRefShortcut = new Regex(@"
            (                               # wrap whole match in $1
              \[
                 ([^\[\]]+)                 # link text = $2; can't contain [ or ]
              \]
            )", RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private string DoAnchors(string text)
        {
            // First, handle reference-style links: [link text] [id]
            text = _anchorRef.Replace(text, new MatchEvaluator(AnchorRefEvaluator));

            // Next, inline-style links: [link text](url "optional title") or [link text](url "optional title")
            text = _anchorInline.Replace(text, new MatchEvaluator(AnchorInlineEvaluator));

            //  Last, handle reference-style shortcuts: [link text]
            //  These must come last in case you've also got [link test][1]
            //  or [link test](/foo)
            text = _anchorRefShortcut.Replace(text, new MatchEvaluator(AnchorRefShortcutEvaluator));
            return text;
        }

        private string AnchorRefEvaluator(Match match)
        {
            string wholeMatch = match.Groups[1].Value;
            string linkText = match.Groups[2].Value;
            string linkID = match.Groups[3].Value.ToLowerInvariant();

            string result;

            // for shortcut links like [this][].
            if (linkID == "")
                linkID = linkText.ToLowerInvariant();

            if (_urls.ContainsKey(linkID))
            {
                string url = _urls[linkID];

                url = EncodeProblemUrlChars(url);
                url = EscapeBoldItalic(url);                
                result = "<a href=\"" + url + "\"";

                if (_titles.ContainsKey(linkID))
                {
                    string title = _titles[linkID];
                    title = EscapeBoldItalic(title);
                    result += " title=\"" + title + "\"";
                }

                result += ">" + linkText + "</a>";
            }
            else
                result = wholeMatch;

            return result;
        }

        private string AnchorRefShortcutEvaluator(Match match)
        {
            string wholeMatch = match.Groups[1].Value;
            string linkText = match.Groups[2].Value;
            string linkID = Regex.Replace(linkText.ToLowerInvariant(), @"[ ]*\n[ ]*", " ");  // lower case and remove newlines / extra spaces

            string result;

            if (_urls.ContainsKey(linkID))
            {
                string url = _urls[linkID];

                url = EncodeProblemUrlChars(url);
                url = EscapeBoldItalic(url);                
                result = "<a href=\"" + url + "\"";

                if (_titles.ContainsKey(linkID))
                {
                    string title = _titles[linkID];
                    title = EscapeBoldItalic(title);
                    result += " title=\"" + title + "\"";
                }

                result += ">" + linkText + "</a>";
            }
            else
                result = wholeMatch;

            return result;
        }


        private string AnchorInlineEvaluator(Match match)
        {
            string linkText = match.Groups[2].Value;
            string url = match.Groups[3].Value;
            string title = match.Groups[6].Value;
            string result;

            url = EncodeProblemUrlChars(url);
            url = EscapeBoldItalic(url);
            if (url.StartsWith("<") && url.EndsWith(">"))
                url = url.Substring(1, url.Length - 2); // remove <>'s surrounding URL, if present            

            result = string.Format("<a href=\"{0}\"", url);

            if (!String.IsNullOrEmpty(title))
            {
                title = title.Replace("\"", "&quot;");
                title = EscapeBoldItalic(title);
                result += string.Format(" title=\"{0}\"", title);
            }

            result += string.Format(">{0}</a>", linkText);
            return result;
        }

        private static Regex _imagesRef = new Regex(@"
                    (               # wrap whole match in $1
                    !\[
                        (.*?)       # alt text = $2
                    \]

                    [ ]?            # one optional space
                    (?:\n[ ]*)?     # one optional newline followed by spaces

                    \[
                        (.*?)       # id = $3
                    \]

                    )", RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);

        private static Regex _imagesInline = new Regex(String.Format(@"
              (                     # wrap whole match in $1
                !\[
                    (.*?)           # alt text = $2
                \]
                \s?                 # one optional whitespace character
                \(                  # literal paren
                    [ ]*
                    ({0})           # href = $3
                    [ ]*
                    (               # $4
                    (['""])       # quote char = $5
                    (.*?)           # title = $6
                    \5              # matching quote
                    [ ]*
                    )?              # title is optional
                \)
              )", GetNestedParensPattern()),
                  RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);

        private string DoImages(string text)
        {
            // First, handle reference-style labeled images: ![alt text][id]
            text = _imagesRef.Replace(text, new MatchEvaluator(ImageReferenceEvaluator));

            // Next, handle inline images:  ![alt text](url "optional title")
            // Don't forget: encode * and _
            text = _imagesInline.Replace(text, new MatchEvaluator(ImageInlineEvaluator));

            return text;
        }

        private string ImageReferenceEvaluator(Match match)
        {
            string wholeMatch = match.Groups[1].Value;
            string altText = match.Groups[2].Value;
            string linkID = match.Groups[3].Value.ToLowerInvariant();
            string result;

            // for shortcut links like ![this][].
            if (linkID == "")
                linkID = altText.ToLowerInvariant();

            altText = altText.Replace("\"", "&quot;");

            if (_urls.ContainsKey(linkID))
            {
                string url = _urls[linkID];
                url = EncodeProblemUrlChars(url);
                url = EscapeBoldItalic(url);                
                result = string.Format("<img src=\"{0}\" alt=\"{1}\"", url, altText);

                if (_titles.ContainsKey(linkID))
                {
                    string title = _titles[linkID];
                    title = EscapeBoldItalic(title);

                    result += string.Format(" title=\"{0}\"", title);
                }

                result += _emptyElementSuffix;
            }
            else
            {
                // If there's no such link ID, leave intact:
                result = wholeMatch;
            }

            return result;
        }

        private string ImageInlineEvaluator(Match match)
        {
            string alt = match.Groups[2].Value;
            string url = match.Groups[3].Value;
            string title = match.Groups[6].Value;
            string result;

            alt = alt.Replace("\"", "&quot;");
            title = title.Replace("\"", "&quot;");
            
            if (url.StartsWith("<") && url.EndsWith(">"))
                url = url.Substring(1, url.Length - 2);    // Remove <>'s surrounding URL, if present
            url = EncodeProblemUrlChars(url);
            url = EscapeBoldItalic(url);

            result = string.Format("<img src=\"{0}\" alt=\"{1}\"", url, alt);

            if (!String.IsNullOrEmpty(title))
            {
                title = EscapeBoldItalic(title);
                result += string.Format(" title=\"{0}\"", title);
            }

            result += _emptyElementSuffix;

            return result;
        }

        private static Regex _headerSetext = new Regex(@"
                ^(.+?)
                [ ]*
                \n
                (=+|-+)     # $1 = string of ='s or -'s
                [ ]*
                \n+",
            RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private static Regex _headerAtx = new Regex(@"
                ^(\#{1,6})  # $1 = string of #'s
                [ ]*
                (.+?)       # $2 = Header text
                [ ]*
                \#*         # optional closing #'s (not counted)
                \n+",
            RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private string DoHeaders(string text)
        {
            text = _headerSetext.Replace(text, new MatchEvaluator(SetextHeaderEvaluator));
            text = _headerAtx.Replace(text, new MatchEvaluator(AtxHeaderEvaluator));
            return text;
        }

        private string SetextHeaderEvaluator(Match match)
        {
            string header = match.Groups[1].Value;
            int level = match.Groups[2].Value.StartsWith("=") ? 1 : 2;
            return string.Format("<h{1}>{0}</h{1}>\n\n", RunSpanGamut(header), level);
        }

        private string AtxHeaderEvaluator(Match match)
        {
            string header = match.Groups[2].Value;
            int level = match.Groups[1].Value.Length;
            return string.Format("<h{1}>{0}</h{1}>\n\n", RunSpanGamut(header), level);
        }


        private static Regex _horizontalRules = new Regex(@"
            ^[ ]{0,3}         # Leading space
                ([-*_])       # $1: First marker
                (?>           # Repeated marker group
                    [ ]{0,2}  # Zero, one, or two spaces.
                    \1        # Marker character
                ){2,}         # Group repeated at least twice
                [ ]*          # Trailing spaces
                $             # End of line.
            ", RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private string DoHorizontalRules(string text)
        {
            return _horizontalRules.Replace(text, "<hr" + _emptyElementSuffix + "\n");
        }

        private static string _wholeList = string.Format(@"
            (                               # $1 = whole list
              (                             # $2
                [ ]{{0,{1}}}
                ({0})                       # $3 = first list item marker
                [ ]+
              )
              (?s:.+?)
              (                             # $4
                  \z
                |
                  \n{{2,}}
                  (?=\S)
                  (?!                       # Negative lookahead for another list item marker
                    [ ]*
                    {0}[ ]+
                  )
              )
            )", string.Format("(?:{0}|{1})", _markerUL, _markerOL), _tabWidth - 1);

        private static Regex _listNested = new Regex(@"^" + _wholeList,
            RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private static Regex _listTopLevel = new Regex(@"(?:(?<=\n\n)|\A\n?)" + _wholeList,
            RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private string DoLists(string text)
        {
            // We use a different prefix before nested lists than top-level lists.
            // See extended comment in _ProcessListItems().
            if (_listLevel > 0)
                text = _listNested.Replace(text, new MatchEvaluator(ListEvaluator));
            else
                text = _listTopLevel.Replace(text, new MatchEvaluator(ListEvaluator));

            return text;
        }

        private string ListEvaluator(Match match)
        {
            string list = match.Groups[1].Value;
            string listType = Regex.IsMatch(match.Groups[3].Value, _markerUL) ? "ul" : "ol";
            string result;

            // Turn double returns into triple returns, so that we can make a
            // paragraph for the last item in a list, if necessary:
            list = Regex.Replace(list, @"\n{2,}", "\n\n\n");
            result = ProcessListItems(list, listType == "ul" ? _markerUL : _markerOL);

            result = string.Format("<{0}>\n{1}</{0}>\n", listType, result);
            return result;
        }

        private string ProcessListItems(string list, string marker)
        {
            // The listLevel global keeps track of when we're inside a list.
            // Each time we enter a list, we increment it; when we leave a list,
            // we decrement. If it's zero, we're not in a list anymore.

            // We do this because when we're not inside a list, we want to treat
            // something like this:

            //    I recommend upgrading to version
            //    8. Oops, now this line is treated
            //    as a sub-list.

            // As a single paragraph, despite the fact that the second line starts
            // with a digit-period-space sequence.

            // Whereas when we're inside a list (or sub-list), that line will be
            // treated as the start of a sub-list. What a kludge, huh? This is
            // an aspect of Markdown's syntax that's hard to parse perfectly
            // without resorting to mind-reading. Perhaps the solution is to
            // change the syntax rules such that sub-lists must start with a
            // starting cardinal number; e.g. "1." or "a.".

            _listLevel++;

            // Trim trailing blank lines:
            list = Regex.Replace(list, @"\n{2,}\z", "\n");

            string pattern = string.Format(
              @"(\n)?                      # leading line = $1
                (^[ ]*)                    # leading whitespace = $2
                ({0}) [ ]+                 # list marker = $3
                ((?s:.+?)                  # list item text = $4
                (\n{{1,2}}))      
                (?= \n* (\z | \2 ({0}) [ ]+))", marker);

            list = Regex.Replace(list, pattern, new MatchEvaluator(ListItemEvaluator),
                                  RegexOptions.IgnorePatternWhitespace | RegexOptions.Multiline);
            _listLevel--;
            return list;
        }

        private string ListItemEvaluator(Match match)
        {
            string item = match.Groups[4].Value;
            string leadingLine = match.Groups[1].Value;

            if (!String.IsNullOrEmpty(leadingLine) || Regex.IsMatch(item, @"\n{2,}"))
                // we could correct any bad indentation here..
                item = RunBlockGamut(Outdent(item) + "\n");
            else
            {
                // recursion for sub-lists
                item = DoLists(Outdent(item));
                item = item.TrimEnd('\n');
                item = RunSpanGamut(item);
            }

            return string.Format("<li>{0}</li>\n", item);
        }


        private static Regex _codeBlock = new Regex(string.Format(@"
                    (?:\n\n|\A\n?)
                    (                        # $1 = the code block -- one or more lines, starting with a space
                    (?:
                        (?:[ ]{{{0}}})       # Lines must start with a tab-width of spaces
                        .*\n+
                    )+
                    )
                    ((?=^[ ]{{0,{0}}}\S)|\Z) # Lookahead for non-space at line-start, or end of doc",
                    _tabWidth), RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled);

        private string DoCodeBlocks(string text)
        {
            text = _codeBlock.Replace(text, new MatchEvaluator(CodeBlockEvaluator));
            return text;
        }

        private string CodeBlockEvaluator(Match match)
        {
            string codeBlock = match.Groups[1].Value;

            codeBlock = EncodeCode(Outdent(codeBlock));
            codeBlock = _newlinesLeadingTrailing.Replace(codeBlock, "");

            return string.Concat("\n\n<pre><code>", codeBlock, "\n</code></pre>\n\n");
        }

        private static Regex _codeSpan = new Regex(@"
                    (?<!\\)   # Character before opening ` can't be a backslash
                    (`+)      # $1 = Opening run of `
                    (.+?)     # $2 = The code block
                    (?<!`)
                    \1
                    (?!`)", RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);

        private string DoCodeSpans(string text)
        {
            //    * You can use multiple backticks as the delimiters if you want to
            //        include literal backticks in the code span. So, this input:
            //
            //        Just type ``foo `bar` baz`` at the prompt.
            //
            //        Will translate to:
            //
            //          <p>Just type <code>foo `bar` baz</code> at the prompt.</p>
            //
            //        There's no arbitrary limit to the number of backticks you
            //        can use as delimters. If you need three consecutive backticks
            //        in your code, use four for delimiters, etc.
            //
            //    * You can use spaces to get literal backticks at the edges:
            //
            //          ... type `` `bar` `` ...
            //
            //        Turns to:
            //
            //          ... type <code>`bar`</code> ...         
            //

            return _codeSpan.Replace(text, new MatchEvaluator(CodeSpanEvaluator));
        }

        private string CodeSpanEvaluator(Match match)
        {
            string span = match.Groups[2].Value;
            span = Regex.Replace(span, @"^[ ]*", ""); // leading whitespace
            span = Regex.Replace(span, @"[ ]*$", ""); // trailing whitespace
            span = EncodeCode(span);

            return string.Concat("<code>", span, "</code>");
        }


        private static Regex _bold = new Regex(@"(\*\*|__) (?=\S) (.+?[*_]*) (?<=\S) \1",
            RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);
        private static Regex _strictBold = new Regex(@"([\W_]|^) (\*\*|__) (?=\S) ([^\r]*?\S[\*_]*) \2 ([\W_]|$)",
            RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);

        private static Regex _italic = new Regex(@"(\*|_) (?=\S) (.+?) (?<=\S) \1",
            RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);
        private static Regex _strictItalic = new Regex(@"([\W_]|^) (\*|_) (?=\S) ([^\r\*_]*?\S) \2 ([\W_]|$)",
            RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline | RegexOptions.Compiled);

        private string DoItalicsAndBold(string text)
        {

            // <strong> must go first, then <em>
            if (_strictBoldItalic)
            {
                text = _strictBold.Replace(text, "$1<strong>$3</strong>$4");
                text = _strictItalic.Replace(text, "$1<em>$3</em>$4");
            }
            else
            {
                text = _bold.Replace(text, "<strong>$2</strong>");
                text = _italic.Replace(text, "<em>$2</em>");
            }
            return text;
        }

        private string DoHardBreaks(string text)
        {
            if (_autoNewlines)
                text = Regex.Replace(text, @"\n", string.Format("<br{0}\n", _emptyElementSuffix));
            else
                text = Regex.Replace(text, @" {2,}\n", string.Format("<br{0}\n", _emptyElementSuffix));
            return text;
        }

        private static Regex _blockquote = new Regex(@"
            (                           # Wrap whole match in $1
                (
                ^[ ]*>[ ]?              # '>' at the start of a line
                    .+\n                # rest of the first line
                (.+\n)*                 # subsequent consecutive lines
                \n*                     # blanks
                )+
            )", RegexOptions.IgnorePatternWhitespace | RegexOptions.Multiline | RegexOptions.Compiled);

        private string DoBlockQuotes(string text)
        {
            return _blockquote.Replace(text, new MatchEvaluator(BlockQuoteEvaluator));
        }

        private string BlockQuoteEvaluator(Match match)
        {
            string bq = match.Groups[1].Value;

            bq = Regex.Replace(bq, @"^[ ]*>[ ]?", "", RegexOptions.Multiline);       // trim one level of quoting
            bq = Regex.Replace(bq, @"^[ ]+$", "", RegexOptions.Multiline);           // trim whitespace-only lines
            bq = RunBlockGamut(bq);                                                  // recurse

            bq = Regex.Replace(bq, @"^", "  ", RegexOptions.Multiline);

            // These leading spaces screw with <pre> content, so we need to fix that:
            bq = Regex.Replace(bq, @"(\s*<pre>.+?</pre>)", new MatchEvaluator(BlockQuoteEvaluator2), RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline);

            return string.Format("<blockquote>\n{0}\n</blockquote>\n\n", bq);
        }

        private string BlockQuoteEvaluator2(Match match)
        {
            return Regex.Replace(match.Groups[1].Value, @"^  ", "", RegexOptions.Multiline);
        }

        private static Regex _autolinkBare = new Regex(@"(^|\s)(https?|ftp)(://[-A-Z0-9+&@#/%?=~_|\[\]\(\)!:,\.;]*[-A-Z0-9+&@#/%=~_|\[\]])($|\W)",
            RegexOptions.IgnoreCase | RegexOptions.Compiled);

        private string DoAutoLinks(string text)
        {

            if (_autoHyperlink)
            {
                // fixup arbitrary URLs by adding Markdown < > so they get linked as well
                // note that at this point, all other URL in the text are already hyperlinked as <a href=""></a>
                // *except* for the <http://www.foo.com> case
                text = _autolinkBare.Replace(text, @"$1<$2$3>$4");
            }

            // Hyperlinks: <http://foo.com>
            text = Regex.Replace(text, "<((https?|ftp):[^'\">\\s]+)>", new MatchEvaluator(HyperlinkEvaluator));

            if (_linkEmails)
            {
                // Email addresses: <address@domain.foo>
                string pattern =
                    @"<
                      (?:mailto:)?
                      (
                        [-.\w]+
                        \@
                        [-a-z0-9]+(\.[-a-z0-9]+)*\.[a-z]+
                      )
                      >";
                text = Regex.Replace(text, pattern, new MatchEvaluator(EmailEvaluator), RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);
            }

            return text;
        }

        private string HyperlinkEvaluator(Match match)
        {
            string link = match.Groups[1].Value;
            return string.Format("<a href=\"{0}\">{0}</a>", link);
        }

        private string EmailEvaluator(Match match)
        {
            string email = Unescape(match.Groups[1].Value);

            //
            //    Input: an email address, e.g. "foo@example.com"
            //
            //    Output: the email address as a mailto link, with each character
            //            of the address encoded as either a decimal or hex entity, in
            //            the hopes of foiling most address harvesting spam bots. E.g.:
            //
            //      <a href="&#x6D;&#97;&#105;&#108;&#x74;&#111;:&#102;&#111;&#111;&#64;&#101;
            //        x&#x61;&#109;&#x70;&#108;&#x65;&#x2E;&#99;&#111;&#109;">&#102;&#111;&#111;
            //        &#64;&#101;x&#x61;&#109;&#x70;&#108;&#x65;&#x2E;&#99;&#111;&#109;</a>
            //
            //    Based by a filter by Matthew Wickline, posted to the BBEdit-Talk
            //    mailing list: <http://tinyurl.com/yu7ue>
            //
            email = "mailto:" + email;

            // leave ':' alone (to spot mailto: later) 
            email = EncodeEmailAddress(email);

            email = string.Format("<a href=\"{0}\">{0}</a>", email);

            // strip the mailto: from the visible part
            email = Regex.Replace(email, "\">.+?:", "\">");
            return email;
        }


        private static Regex _outDent = new Regex(@"^[ ]{1," + _tabWidth + @"}", RegexOptions.Multiline | RegexOptions.Compiled);

        private string Outdent(string block)
        {
            return _outDent.Replace(block, "");
        }


        #region Encoding and Normalization


        private string EncodeEmailAddress(string addr)
        {
            var sb = new StringBuilder(addr.Length * 5);
            var rand = new Random();
            int r;
            foreach (char c in addr)
            {
                r = rand.Next(1, 100);
                if ((r > 90 || c == ':') && c != '@')
                    sb.Append(c);                         // m
                else if (r < 45)
                    sb.AppendFormat("&#x{0:x};", (int)c); // &#x6D
                else
                    sb.AppendFormat("&#{0};", (int)c);    // &#109
            }
            return sb.ToString();
        }

        private static Regex _codeEncoder = new Regex(@"&|<|>|\\|\*|_|\{|\}|\[|\]", RegexOptions.Compiled);

        private string EncodeCode(string code)
        {
            return _codeEncoder.Replace(code, EncodeCodeEvaluator);
        }
        private string EncodeCodeEvaluator(Match match)
        {
            switch (match.Value)
            {
                // Encode all ampersands; HTML entities are not
                // entities within a Markdown code span.
                case "&":
                    return "&amp;";
                // Do the angle bracket song and dance
                case "<":
                    return "&lt;";
                case ">":
                    return "&gt;";
                // escape characters that are magic in Markdown
                default:
                    return _escapeTable[match.Value];
            }
        }


        private static Regex _amps = new Regex(@"&(?!(#[0-9]+)|(#[xX][a-fA-F0-9])|([a-zA-Z][a-zA-Z0-9]*);)", RegexOptions.ExplicitCapture | RegexOptions.Compiled);
        private static Regex _angles = new Regex(@"<(?![A-Za-z/?\$!])", RegexOptions.ExplicitCapture | RegexOptions.Compiled);

        private string EncodeAmpsAndAngles(string s)
        {
            s = _amps.Replace(s, "&amp;");
            s = _angles.Replace(s, "&lt;");
            return s;
        }

        private static Regex _backslashEscapes; 

        private string EscapeBackslashes(string s)
        {
            return _backslashEscapes.Replace(s, new MatchEvaluator(EscapeBackslashesEvaluator));
        }
        private string EscapeBackslashesEvaluator(Match match)
        {
            return _backslashEscapeTable[match.Value];
        }
       
        private static Regex _unescapes = new Regex("\x1A\\d+\x1A", RegexOptions.Compiled);

        private string Unescape(string s)
        {
            return _unescapes.Replace(s, new MatchEvaluator(UnescapeEvaluator));
        }
        private string UnescapeEvaluator(Match match)
        {
            return _invertedEscapeTable[match.Value];
        }


        private string EscapeBoldItalic(string s)
        {
            s = s.Replace("*", _escapeTable["*"]);
            s = s.Replace("_", _escapeTable["_"]);
            return s;
        }

        private static char[] _problemUrlChars = @"""'*()[]$:".ToCharArray();

        private string EncodeProblemUrlChars(string url)
        {
            if (!_encodeProblemUrlCharacters) return url;

            var sb = new StringBuilder(url.Length);
            bool encode;
            char c;

            for (int i = 0; i < url.Length; i++)
            {
                c = url[i];
                encode = Array.IndexOf(_problemUrlChars, c) != -1;
                if (encode && c == ':' && i < url.Length - 1)
                    encode = !(url[i + 1] == '/') && !(url[i + 1] >= '0' && url[i + 1] <= '9');

                if (encode)
                    sb.Append("%" + String.Format("{0:x}", (byte)c));
                else
                    sb.Append(c);                
            }

            return sb.ToString();
        }


        private string EscapeSpecialCharsWithinTagAttributes(string text)
        {
            var tokens = TokenizeHTML(text);

            // now, rebuild text from the tokens
            var sb = new StringBuilder(text.Length);

            foreach (var token in tokens)
            {
                string value = token.Value;

                if (token.Type == TokenType.Tag)
                {
                    value = value.Replace(@"\", _escapeTable[@"\"]);
                    value = Regex.Replace(value, "(?<=.)</?code>(?=.)", _escapeTable[@"`"]);
                    value = EscapeBoldItalic(value);
                }

                sb.Append(value);
            }

            return sb.ToString();
        }

        private string Normalize(string text)
        {            
            var output = new StringBuilder(text.Length);
            var line = new StringBuilder();
            bool valid = false;

            for (int i = 0; i < text.Length; i++)
            {
                switch (text[i])
                {
                    case '\n':
                        if (valid) output.Append(line);
                        output.Append('\n');
                        line.Length = 0; valid = false;
                        break;
                    case '\r':
                        if ((i < text.Length - 1) && (text[i + 1] != '\n'))
                        {
                            if (valid) output.Append(line);
                            output.Append('\n');
                            line.Length = 0; valid = false;
                        }
                        break;
                    case '\t':
                        int width = (_tabWidth - line.Length % _tabWidth);
                        for (int k = 0; k < width; k++)
                            line.Append(' ');
                        break;
                    case '\x1A':
                        break;
                    default:
                        if (!valid && text[i] != ' ') valid = true;
                        line.Append(text[i]);
                        break;
                }
            }

            if (valid) output.Append(line);
            output.Append('\n');

            // add two newlines to the end before return
            return output.Append("\n\n").ToString();
        }

        #endregion

        private static string RepeatString(string text, int count)
        {
            var sb = new StringBuilder(text.Length * count);
            for (int i = 0; i < count; i++)
                sb.Append(text);
            return sb.ToString();
        }

    }
}