pkgs/markdown/lib/src/inline_parser.dart - tools.git - Git at Google

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 library markdown.src.inline_parser;

 import 'ast.dart';
 import 'document.dart';
 import 'util.dart';

 /// Maintains the internal state needed to parse inline span elements in
 /// markdown.
 class InlineParser {
   static final List<InlineSyntax> _defaultSyntaxes = <InlineSyntax>[
     // This first regexp matches plain text to accelerate parsing.  It must
     // be written so that it does not match any prefix of any following
     // syntax.  Most markdown is plain text, so it is faster to match one
     // regexp per 'word' rather than fail to match all the following regexps
     // at each non-syntax character position.  It is much more important
     // that the regexp is fast than complete (for example, adding grouping
     // is likely to slow the regexp down enough to negate its benefit).
     // Since it is purely for optimization, it can be removed for debugging.

     // TODO(amouravski): this regex will glom up any custom syntaxes unless
     // they're at the beginning.
     new TextSyntax(r'\s*[A-Za-z0-9]+'),

     // The real syntaxes.
     new AutolinkSyntax(),
     new LinkSyntax(),
     new ImageLinkSyntax(),
     // "*" surrounded by spaces is left alone.
     new TextSyntax(r' \* '),
     // "_" surrounded by spaces is left alone.
     new TextSyntax(r' _ '),
     // Leave already-encoded HTML entities alone. Ensures we don't turn
     // "&amp;" into "&amp;amp;"
     new TextSyntax(r'&[#a-zA-Z0-9]*;'),
     // Encode "&".
     new TextSyntax(r'&', sub: '&amp;'),
     // Encode "<". (Why not encode ">" too? Gruber is toying with us.)
     new TextSyntax(r'<', sub: '&lt;'),
     // Parse "**strong**" tags.
     new TagSyntax(r'\*\*', tag: 'strong'),
     // Parse "__strong__" tags.
     new TagSyntax(r'__', tag: 'strong'),
     // Parse "*emphasis*" tags.
     new TagSyntax(r'\*', tag: 'em'),
     // Parse "_emphasis_" tags.
     // TODO(rnystrom): Underscores in the middle of a word should not be
     // parsed as emphasis like_in_this.
     new TagSyntax(r'_', tag: 'em'),
     // Parse inline code within double backticks: "``code``".
     new CodeSyntax(r'``\s?((?:.|\n)*?)\s?``'),
     // Parse inline code within backticks: "`code`".
     new CodeSyntax(r'`([^`]*)`')
     // We will add the LinkSyntax once we know about the specific link resolver.
   ];

   /// The string of markdown being parsed.
   final String source;

   /// The markdown document this parser is parsing.
   final Document document;

   final List<InlineSyntax> syntaxes = <InlineSyntax>[];

   /// The current read position.
   int pos = 0;

   /// Starting position of the last unconsumed text.
   int start = 0;

   final List<TagState> _stack;

   InlineParser(this.source, this.document) : _stack = <TagState>[] {
     // User specified syntaxes are the first syntaxes to be evaluated.
     if (document.inlineSyntaxes != null) {
       syntaxes.addAll(document.inlineSyntaxes);
     }

     syntaxes.addAll(_defaultSyntaxes);

     // Custom link resolvers goes after the generic text syntax.
     syntaxes.insertAll(1, [
       new LinkSyntax(linkResolver: document.linkResolver),
       new ImageLinkSyntax(linkResolver: document.imageLinkResolver)
     ]);
   }

   List<Node> parse() {
     // Make a fake top tag to hold the results.
     _stack.add(new TagState(0, 0, null));

     while (!isDone) {
       var matched = false;

       // See if any of the current tags on the stack match. We don't allow tags
       // of the same kind to nest, so this takes priority over other possible
       // matches.
       for (var i = _stack.length - 1; i > 0; i--) {
         if (_stack[i].tryMatch(this)) {
           matched = true;
           break;
         }
       }

       if (matched) continue;

       // See if the current text matches any defined markdown syntax.
       for (var syntax in syntaxes) {
         if (syntax.tryMatch(this)) {
           matched = true;
           break;
         }
       }

       if (matched) continue;

       // If we got here, it's just text.
       advanceBy(1);
     }

     // Unwind any unmatched tags and get the results.
     return _stack[0].close(this, null);
   }

   void writeText() {
     writeTextRange(start, pos);
     start = pos;
   }

   void writeTextRange(int start, int end) {
     if (end <= start) return;

     var text = source.substring(start, end);
     var nodes = _stack.last.children;

     // If the previous node is text too, just append.
     if (nodes.length > 0 && nodes.last is Text) {
       nodes[nodes.length - 1] = new Text('${nodes.last.text}$text');
     } else {
       nodes.add(new Text(text));
     }
   }

   void addNode(Node node) {
     _stack.last.children.add(node);
   }

   // TODO(rnystrom): Only need this because RegExp doesn't let you start
   // searching from a given offset.
   @deprecated
   String get currentSource => source.substring(pos, source.length);

   bool get isDone => pos == source.length;

   void advanceBy(int length) {
     pos += length;
   }

   void consume(int length) {
     pos += length;
     start = pos;
   }
 }

 /// Represents one kind of markdown tag that can be parsed.
 abstract class InlineSyntax {
   final RegExp pattern;

   InlineSyntax(String pattern) : pattern = new RegExp(pattern, multiLine: true);

   bool tryMatch(InlineParser parser) {
     var startMatch = pattern.matchAsPrefix(parser.source, parser.pos);
     if (startMatch != null) {
       // Write any existing plain text up to this point.
       parser.writeText();

       if (onMatch(parser, startMatch)) parser.consume(startMatch[0].length);
       return true;
     }

     return false;
   }

   bool onMatch(InlineParser parser, Match match);
 }

 /// Matches stuff that should just be passed through as straight text.
 class TextSyntax extends InlineSyntax {
   final String substitute;

   TextSyntax(String pattern, {String sub})
       : super(pattern),
         substitute = sub;

   bool onMatch(InlineParser parser, Match match) {
     if (substitute == null) {
       // Just use the original matched text.
       parser.advanceBy(match[0].length);
       return false;
     }

     // Insert the substitution.
     parser.addNode(new Text(substitute));
     return true;
   }
 }

 /// Matches autolinks like `<http://foo.com>`.
 class AutolinkSyntax extends InlineSyntax {
   AutolinkSyntax() : super(r'<((http|https|ftp)://[^>]*)>');
   // TODO(rnystrom): Make case insensitive.

   bool onMatch(InlineParser parser, Match match) {
     var url = match[1];
     var anchor = new Element.text('a', escapeHtml(url));
     anchor.attributes['href'] = url;
     parser.addNode(anchor);

     return true;
   }
 }

 /// Matches syntax that has a pair of tags and becomes an element, like `*` for
 /// `<em>`. Allows nested tags.
 class TagSyntax extends InlineSyntax {
   final RegExp endPattern;
   final String tag;

   TagSyntax(String pattern, {this.tag, String end})
       : super(pattern),
         endPattern = new RegExp((end != null) ? end : pattern, multiLine: true);

   bool onMatch(InlineParser parser, Match match) {
     parser._stack
         .add(new TagState(parser.pos, parser.pos + match[0].length, this));
     return true;
   }

   bool onMatchEnd(InlineParser parser, Match match, TagState state) {
     parser.addNode(new Element(tag, state.children));
     return true;
   }
 }

 /// Matches inline links like `[blah] [id]` and `[blah] (url)`.
 class LinkSyntax extends TagSyntax {
   final Resolver linkResolver;

   /// Weather or not this link was resolved by a [Resolver]
   bool resolved = false;

   /// The regex for the end of a link needs to handle both reference style and
   /// inline styles as well as optional titles for inline links. To make that
   /// a bit more palatable, this breaks it into pieces.
   static get linkPattern {
     var refLink = r'\s?\[([^\]]*)\]'; // "[id]" reflink id.
     var title = r'(?:[ ]*"([^"]+)"|)'; // Optional title in quotes.
     var inlineLink = '\\s?\\(([^ )]+)$title\\)'; // "(url "title")" link.
     return '\](?:($refLink|$inlineLink)|)';

     // The groups matched by this are:
     // 1: Will be non-empty if it's either a ref or inline link. Will be empty
     //    if it's just a bare pair of square brackets with nothing after them.
     // 2: Contains the id inside [] for a reference-style link.
     // 3: Contains the URL for an inline link.
     // 4: Contains the title, if present, for an inline link.
   }

   LinkSyntax({this.linkResolver, String pattern: r'\['})
       : super(pattern, end: linkPattern);

   Node createNode(InlineParser parser, Match match, TagState state) {
     // If we didn't match refLink or inlineLink, then it means there was
     // nothing after the first square bracket, so it isn't a normal markdown
     // link at all. Instead, we allow users of the library to specify a special
     // resolver function ([linkResolver]) that may choose to handle
     // this. Otherwise, it's just treated as plain text.
     if (match[1] == null) {
       if (linkResolver == null) return null;

       // Treat the contents as unparsed text even if they happen to match. This
       // way, we can handle things like [LINK_WITH_UNDERSCORES] as a link and
       // not get confused by the emphasis.
       var textToResolve = parser.source.substring(state.endPos, parser.pos);

       // See if we have a resolver that will generate a link for us.
       resolved = true;
       return linkResolver(textToResolve);
     } else {
       var link = getLink(parser, match, state);
       if (link == null) return null;

       var node = new Element('a', state.children);

       node.attributes["href"] = escapeHtml(link.url);
       if (link.title != null) node.attributes['title'] = escapeHtml(link.title);

       return node;
     }
   }

   Link getLink(InlineParser parser, Match match, TagState state) {
     if (match[3] != null && match[3] != '') {
       // Inline link like [foo](url).
       var url = match[3];
       var title = match[4];

       // For whatever reason, markdown allows angle-bracketed URLs here.
       if (url.startsWith('<') && url.endsWith('>')) {
         url = url.substring(1, url.length - 1);
       }

       return new Link(null, url, title);
     } else {
       var id;
       // Reference link like [foo] [bar].
       if (match[2] == '') {
         // The id is empty ("[]") so infer it from the contents.
         id = parser.source.substring(state.startPos + 1, parser.pos);
       } else {
         id = match[2];
       }

       // References are case-insensitive.
       id = id.toLowerCase();
       return parser.document.refLinks[id];
     }
   }

   bool onMatchEnd(InlineParser parser, Match match, TagState state) {
     var node = createNode(parser, match, state);
     if (node == null) return false;

     parser.addNode(node);
     return true;
   }
 }

 /// Matches images like `![alternate text](url "optional title")` and
 /// `![alternate text][url reference]`.
 class ImageLinkSyntax extends LinkSyntax {
   final Resolver linkResolver;

   ImageLinkSyntax({this.linkResolver}) : super(pattern: r'!\[');

   Node createNode(InlineParser parser, Match match, TagState state) {
     var node = super.createNode(parser, match, state);

     if (resolved) return node;
     if (node == null) return null;

     var imageElement = new Element.withTag("img");
     imageElement.attributes["src"] = node.attributes["href"];

     if (node.attributes.containsKey("title")) {
       imageElement.attributes["title"] = node.attributes["title"];
     }

     var alt = node.children.map((e) => e is! Text ? '' : e.text).join(" ");
     if (alt != "") imageElement.attributes["alt"] = alt;

     node.children
       ..clear()
       ..add(imageElement);

     return node;
   }
 }

 /// Matches backtick-enclosed inline code blocks.
 class CodeSyntax extends InlineSyntax {
   CodeSyntax(String pattern) : super(pattern);

   bool onMatch(InlineParser parser, Match match) {
     parser.addNode(new Element.text('code', escapeHtml(match[1])));
     return true;
   }
 }

 /// Keeps track of a currently open tag while it is being parsed. The parser
 /// maintains a stack of these so it can handle nested tags.
 class TagState {
   /// The point in the original source where this tag started.
   final int startPos;

   /// The point in the original source where open tag ended.
   final int endPos;

   /// The syntax that created this node.
   final TagSyntax syntax;

   /// The children of this node. Will be `null` for text nodes.
   final List<Node> children;

   TagState(this.startPos, this.endPos, this.syntax) : children = <Node>[];

   /// Attempts to close this tag by matching the current text against its end
   /// pattern.
   bool tryMatch(InlineParser parser) {
     var endMatch = syntax.endPattern.matchAsPrefix(parser.source, parser.pos);
     if (endMatch != null) {
       // Close the tag.
       close(parser, endMatch);
       return true;
     }

     return false;
   }

   /// Pops this tag off the stack, completes it, and adds it to the output.
   /// Will discard any unmatched tags that happen to be above it on the stack.
   /// If this is the last node in the stack, returns its children.
   List<Node> close(InlineParser parser, Match endMatch) {
     // If there are unclosed tags on top of this one when it's closed, that
     // means they are mismatched. Mismatched tags are treated as plain text in
     // markdown. So for each tag above this one, we write its start tag as text
     // and then adds its children to this one's children.
     var index = parser._stack.indexOf(this);

     // Remove the unmatched children.
     var unmatchedTags = parser._stack.sublist(index + 1);
     parser._stack.removeRange(index + 1, parser._stack.length);

     // Flatten them out onto this tag.
     for (var unmatched in unmatchedTags) {
       // Write the start tag as text.
       parser.writeTextRange(unmatched.startPos, unmatched.endPos);

       // Bequeath its children unto this tag.
       children.addAll(unmatched.children);
     }

     // Pop this off the stack.
     parser.writeText();
     parser._stack.removeLast();

     // If the stack is empty now, this is the special "results" node.
     if (parser._stack.length == 0) return children;

     // We are still parsing, so add this to its parent's children.
     if (syntax.onMatchEnd(parser, endMatch, this)) {
       parser.consume(endMatch[0].length);
     } else {
       // Didn't close correctly so revert to text.
       parser.start = startPos;
       parser.advanceBy(endMatch[0].length);
     }

     return null;
   }
 }
	// Copyright (c) 2012, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	library markdown.src.inline_parser;

	import 'ast.dart';
	import 'document.dart';
	import 'util.dart';

	/// Maintains the internal state needed to parse inline span elements in
	/// markdown.
	class InlineParser {
	static final List<InlineSyntax> _defaultSyntaxes = <InlineSyntax>[
	// This first regexp matches plain text to accelerate parsing. It must
	// be written so that it does not match any prefix of any following
	// syntax. Most markdown is plain text, so it is faster to match one
	// regexp per 'word' rather than fail to match all the following regexps
	// at each non-syntax character position. It is much more important
	// that the regexp is fast than complete (for example, adding grouping
	// is likely to slow the regexp down enough to negate its benefit).
	// Since it is purely for optimization, it can be removed for debugging.

	// TODO(amouravski): this regex will glom up any custom syntaxes unless
	// they're at the beginning.
	new TextSyntax(r'\s*[A-Za-z0-9]+'),

	// The real syntaxes.
	new AutolinkSyntax(),
	new LinkSyntax(),
	new ImageLinkSyntax(),
	// "*" surrounded by spaces is left alone.
	new TextSyntax(r' \* '),
	// "_" surrounded by spaces is left alone.
	new TextSyntax(r' _ '),
	// Leave already-encoded HTML entities alone. Ensures we don't turn
	// "&" into "&amp;"
	new TextSyntax(r'&[#a-zA-Z0-9]*;'),
	// Encode "&".
	new TextSyntax(r'&', sub: '&'),
	// Encode "<". (Why not encode ">" too? Gruber is toying with us.)
	new TextSyntax(r'<', sub: '<'),
	// Parse "strong" tags.
	new TagSyntax(r'\\', tag: 'strong'),
	// Parse "__strong__" tags.
	new TagSyntax(r'__', tag: 'strong'),
	// Parse "emphasis" tags.
	new TagSyntax(r'\*', tag: 'em'),
	// Parse "_emphasis_" tags.
	// TODO(rnystrom): Underscores in the middle of a word should not be
	// parsed as emphasis like_in_this.
	new TagSyntax(r'_', tag: 'em'),
	// Parse inline code within double backticks: "``code``".
	new CodeSyntax(r'``\s?((?:.\|\n)*?)\s?``'),
	// Parse inline code within backticks: "`code`".
	new CodeSyntax(r'`([^`]*)`')
	// We will add the LinkSyntax once we know about the specific link resolver.
	];

	/// The string of markdown being parsed.
	final String source;

	/// The markdown document this parser is parsing.
	final Document document;

	final List<InlineSyntax> syntaxes = <InlineSyntax>[];

	/// The current read position.
	int pos = 0;

	/// Starting position of the last unconsumed text.
	int start = 0;

	final List<TagState> _stack;

	InlineParser(this.source, this.document) : _stack = <TagState>[] {
	// User specified syntaxes are the first syntaxes to be evaluated.
	if (document.inlineSyntaxes != null) {
	syntaxes.addAll(document.inlineSyntaxes);
	}

	syntaxes.addAll(_defaultSyntaxes);

	// Custom link resolvers goes after the generic text syntax.
	syntaxes.insertAll(1, [
	new LinkSyntax(linkResolver: document.linkResolver),
	new ImageLinkSyntax(linkResolver: document.imageLinkResolver)
	]);
	}

	List<Node> parse() {
	// Make a fake top tag to hold the results.
	_stack.add(new TagState(0, 0, null));

	while (!isDone) {
	var matched = false;

	// See if any of the current tags on the stack match. We don't allow tags
	// of the same kind to nest, so this takes priority over other possible
	// matches.
	for (var i = _stack.length - 1; i > 0; i--) {
	if (_stack[i].tryMatch(this)) {
	matched = true;
	break;
	}
	}

	if (matched) continue;

	// See if the current text matches any defined markdown syntax.
	for (var syntax in syntaxes) {
	if (syntax.tryMatch(this)) {
	matched = true;
	break;
	}
	}

	if (matched) continue;

	// If we got here, it's just text.
	advanceBy(1);
	}

	// Unwind any unmatched tags and get the results.
	return _stack[0].close(this, null);
	}

	void writeText() {
	writeTextRange(start, pos);
	start = pos;
	}

	void writeTextRange(int start, int end) {
	if (end <= start) return;

	var text = source.substring(start, end);
	var nodes = _stack.last.children;

	// If the previous node is text too, just append.
	if (nodes.length > 0 && nodes.last is Text) {
	nodes[nodes.length - 1] = new Text('${nodes.last.text}$text');
	} else {
	nodes.add(new Text(text));
	}
	}

	void addNode(Node node) {
	_stack.last.children.add(node);
	}

	// TODO(rnystrom): Only need this because RegExp doesn't let you start
	// searching from a given offset.
	@deprecated
	String get currentSource => source.substring(pos, source.length);

	bool get isDone => pos == source.length;

	void advanceBy(int length) {
	pos += length;
	}

	void consume(int length) {
	pos += length;
	start = pos;
	}
	}

	/// Represents one kind of markdown tag that can be parsed.
	abstract class InlineSyntax {
	final RegExp pattern;

	InlineSyntax(String pattern) : pattern = new RegExp(pattern, multiLine: true);

	bool tryMatch(InlineParser parser) {
	var startMatch = pattern.matchAsPrefix(parser.source, parser.pos);
	if (startMatch != null) {
	// Write any existing plain text up to this point.
	parser.writeText();

	if (onMatch(parser, startMatch)) parser.consume(startMatch[0].length);
	return true;
	}

	return false;
	}

	bool onMatch(InlineParser parser, Match match);
	}

	/// Matches stuff that should just be passed through as straight text.
	class TextSyntax extends InlineSyntax {
	final String substitute;

	TextSyntax(String pattern, {String sub})
	: super(pattern),
	substitute = sub;

	bool onMatch(InlineParser parser, Match match) {
	if (substitute == null) {
	// Just use the original matched text.
	parser.advanceBy(match[0].length);
	return false;
	}

	// Insert the substitution.
	parser.addNode(new Text(substitute));
	return true;
	}
	}

	/// Matches autolinks like `<http://foo.com>`.
	class AutolinkSyntax extends InlineSyntax {
	AutolinkSyntax() : super(r'<((http\|https\|ftp)://[^>]*)>');
	// TODO(rnystrom): Make case insensitive.

	bool onMatch(InlineParser parser, Match match) {
	var url = match[1];
	var anchor = new Element.text('a', escapeHtml(url));
	anchor.attributes['href'] = url;
	parser.addNode(anchor);

	return true;
	}
	}

	/// Matches syntax that has a pair of tags and becomes an element, like `*` for
	/// `<em>`. Allows nested tags.
	class TagSyntax extends InlineSyntax {
	final RegExp endPattern;
	final String tag;

	TagSyntax(String pattern, {this.tag, String end})
	: super(pattern),
	endPattern = new RegExp((end != null) ? end : pattern, multiLine: true);

	bool onMatch(InlineParser parser, Match match) {
	parser._stack
	.add(new TagState(parser.pos, parser.pos + match[0].length, this));
	return true;
	}

	bool onMatchEnd(InlineParser parser, Match match, TagState state) {
	parser.addNode(new Element(tag, state.children));
	return true;
	}
	}

	/// Matches inline links like `[blah] [id]` and `[blah] (url)`.
	class LinkSyntax extends TagSyntax {
	final Resolver linkResolver;

	/// Weather or not this link was resolved by a [Resolver]
	bool resolved = false;

	/// The regex for the end of a link needs to handle both reference style and
	/// inline styles as well as optional titles for inline links. To make that
	/// a bit more palatable, this breaks it into pieces.
	static get linkPattern {
	var refLink = r'\s?\[([^\]]*)\]'; // "[id]" reflink id.
	var title = r'(?:[ ]*"([^"]+)"\|)'; // Optional title in quotes.
	var inlineLink = '\\s?\\(([^ )]+)$title\\)'; // "(url "title")" link.
	return '\](?:($refLink\|$inlineLink)\|)';

	// The groups matched by this are:
	// 1: Will be non-empty if it's either a ref or inline link. Will be empty
	// if it's just a bare pair of square brackets with nothing after them.
	// 2: Contains the id inside [] for a reference-style link.
	// 3: Contains the URL for an inline link.
	// 4: Contains the title, if present, for an inline link.
	}

	LinkSyntax({this.linkResolver, String pattern: r'\['})
	: super(pattern, end: linkPattern);

	Node createNode(InlineParser parser, Match match, TagState state) {
	// If we didn't match refLink or inlineLink, then it means there was
	// nothing after the first square bracket, so it isn't a normal markdown
	// link at all. Instead, we allow users of the library to specify a special
	// resolver function ([linkResolver]) that may choose to handle
	// this. Otherwise, it's just treated as plain text.
	if (match[1] == null) {
	if (linkResolver == null) return null;

	// Treat the contents as unparsed text even if they happen to match. This
	// way, we can handle things like [LINK_WITH_UNDERSCORES] as a link and
	// not get confused by the emphasis.
	var textToResolve = parser.source.substring(state.endPos, parser.pos);

	// See if we have a resolver that will generate a link for us.
	resolved = true;
	return linkResolver(textToResolve);
	} else {
	var link = getLink(parser, match, state);
	if (link == null) return null;

	var node = new Element('a', state.children);

	node.attributes["href"] = escapeHtml(link.url);
	if (link.title != null) node.attributes['title'] = escapeHtml(link.title);

	return node;
	}
	}

	Link getLink(InlineParser parser, Match match, TagState state) {
	if (match[3] != null && match[3] != '') {
	// Inline link like [foo](url).
	var url = match[3];
	var title = match[4];

	// For whatever reason, markdown allows angle-bracketed URLs here.
	if (url.startsWith('<') && url.endsWith('>')) {
	url = url.substring(1, url.length - 1);
	}

	return new Link(null, url, title);
	} else {
	var id;
	// Reference link like [foo] [bar].
	if (match[2] == '') {
	// The id is empty ("[]") so infer it from the contents.
	id = parser.source.substring(state.startPos + 1, parser.pos);
	} else {
	id = match[2];
	}

	// References are case-insensitive.
	id = id.toLowerCase();
	return parser.document.refLinks[id];
	}
	}

	bool onMatchEnd(InlineParser parser, Match match, TagState state) {
	var node = createNode(parser, match, state);
	if (node == null) return false;

	parser.addNode(node);
	return true;
	}
	}

	/// Matches images like `![alternate text](url "optional title")` and
	/// `![alternate text][url reference]`.
	class ImageLinkSyntax extends LinkSyntax {
	final Resolver linkResolver;

	ImageLinkSyntax({this.linkResolver}) : super(pattern: r'!\[');

	Node createNode(InlineParser parser, Match match, TagState state) {
	var node = super.createNode(parser, match, state);

	if (resolved) return node;
	if (node == null) return null;

	var imageElement = new Element.withTag("img");
	imageElement.attributes["src"] = node.attributes["href"];

	if (node.attributes.containsKey("title")) {
	imageElement.attributes["title"] = node.attributes["title"];
	}

	var alt = node.children.map((e) => e is! Text ? '' : e.text).join(" ");
	if (alt != "") imageElement.attributes["alt"] = alt;

	node.children
	..clear()
	..add(imageElement);

	return node;
	}
	}

	/// Matches backtick-enclosed inline code blocks.
	class CodeSyntax extends InlineSyntax {
	CodeSyntax(String pattern) : super(pattern);

	bool onMatch(InlineParser parser, Match match) {
	parser.addNode(new Element.text('code', escapeHtml(match[1])));
	return true;
	}
	}

	/// Keeps track of a currently open tag while it is being parsed. The parser
	/// maintains a stack of these so it can handle nested tags.
	class TagState {
	/// The point in the original source where this tag started.
	final int startPos;

	/// The point in the original source where open tag ended.
	final int endPos;

	/// The syntax that created this node.
	final TagSyntax syntax;

	/// The children of this node. Will be `null` for text nodes.
	final List<Node> children;

	TagState(this.startPos, this.endPos, this.syntax) : children = <Node>[];

	/// Attempts to close this tag by matching the current text against its end
	/// pattern.
	bool tryMatch(InlineParser parser) {
	var endMatch = syntax.endPattern.matchAsPrefix(parser.source, parser.pos);
	if (endMatch != null) {
	// Close the tag.
	close(parser, endMatch);
	return true;
	}

	return false;
	}

	/// Pops this tag off the stack, completes it, and adds it to the output.
	/// Will discard any unmatched tags that happen to be above it on the stack.
	/// If this is the last node in the stack, returns its children.
	List<Node> close(InlineParser parser, Match endMatch) {
	// If there are unclosed tags on top of this one when it's closed, that
	// means they are mismatched. Mismatched tags are treated as plain text in
	// markdown. So for each tag above this one, we write its start tag as text
	// and then adds its children to this one's children.
	var index = parser._stack.indexOf(this);

	// Remove the unmatched children.
	var unmatchedTags = parser._stack.sublist(index + 1);
	parser._stack.removeRange(index + 1, parser._stack.length);

	// Flatten them out onto this tag.
	for (var unmatched in unmatchedTags) {
	// Write the start tag as text.
	parser.writeTextRange(unmatched.startPos, unmatched.endPos);

	// Bequeath its children unto this tag.
	children.addAll(unmatched.children);
	}

	// Pop this off the stack.
	parser.writeText();
	parser._stack.removeLast();

	// If the stack is empty now, this is the special "results" node.
	if (parser._stack.length == 0) return children;

	// We are still parsing, so add this to its parent's children.
	if (syntax.onMatchEnd(parser, endMatch, this)) {
	parser.consume(endMatch[0].length);
	} else {
	// Didn't close correctly so revert to text.
	parser.start = startPos;
	parser.advanceBy(endMatch[0].length);
	}

	return null;
	}
	}