Pandoc-style fenced block syntax.

pkgs/markdown/lib/src/markdown/block_parser.dart

diff --git a/pkgs/markdown/lib/src/markdown/block_parser.dart b/pkgs/markdown/lib/src/markdown/block_parser.dart
index 42b9353..14ddf86 100644
--- a/pkgs/markdown/lib/src/markdown/block_parser.dart
+++ b/pkgs/markdown/lib/src/markdown/block_parser.dart
@@ -22,6 +22,9 @@
 /// GitHub style triple quoted code block.
 final _RE_CODE = new RegExp(r'^```(.*)$');
 
+/// Pandoc style code block.
+final _RE_PANDOC_CODE = new RegExp(r'^(`{3,}|~{3,})(.*)$');
+
 /// Three or more hyphens, asterisks or underscores by themselves. Note that
 /// a line like `----` is valid as both HR and SETEXT. In case of a tie,
 /// SETEXT should win.
@@ -98,6 +101,7 @@
           new HeaderSyntax(),
           new CodeBlockSyntax(),
           new GitHubCodeBlockSyntax(),
+          new PandocCodeBlockSyntax(),
           new BlockquoteSyntax(),
           new HorizontalRuleSyntax(),
           new UnorderedListSyntax(),
@@ -280,6 +284,53 @@
   }
 }
 
+/// Parses preformatted code blocks between two ~~~ or ``` sequences.
+/// [Pandoc's markdown documentation](http://johnmacfarlane.net/pandoc/demo/example9/pandocs-markdown.html).
+class PandocCodeBlockSyntax extends BlockSyntax {
+  RegExp get pattern => _RE_PANDOC_CODE;
+
+  List<String> parseChildLines(BlockParser parser, String endBlock) {
+    final childLines = <String>[];
+    parser.advance();
+    while (!parser.isDone) {
+      var match = pattern.firstMatch(parser.current);
+      if (match == null || !match[1].startsWith(endBlock)) {
+        childLines.add(parser.current);
+        parser.advance();
+      } else {
+        parser.advance();
+        break;
+      }
+    }
+    return childLines;
+  }
+
+  Node parse(BlockParser parser) {
+    // Get the syntax identifier, if there is one.
+    var match = pattern.firstMatch(parser.current);
+    var endBlock = match.group(1);
+    var syntax = match.group(2);
+
+    final childLines = parseChildLines(parser, endBlock);
+
+    // The Markdown tests expect a trailing newline.
+    childLines.add('');
+
+    // Escape the code.
+    final escaped = escapeHtml(childLines.join('\n'));
+
+    var element = new Element('pre', [new Element.text('code', escaped)]);
+    if (syntax != '') {
+      element.attributes['class'] = syntax;
+    }
+    return element;
+  }
+
+  void _addElementAttributes(Element element) {
+
+  }
+}
+
 /// Parses horizontal rules like `---`, `_ _ _`, `*  *  *`, etc.
 class HorizontalRuleSyntax extends BlockSyntax {
   RegExp get pattern => _RE_HR;

pkgs/markdown/test/markdown_test.dart

diff --git a/pkgs/markdown/test/markdown_test.dart b/pkgs/markdown/test/markdown_test.dart
index fcf76d1..6416278 100644
--- a/pkgs/markdown/test/markdown_test.dart
+++ b/pkgs/markdown/test/markdown_test.dart
@@ -451,6 +451,37 @@
         <pre><code>&lt;&amp;&gt;
         </code></pre>
         ''');
+
+    validate('Pandoc style without language identifier', '''
+        ~~~~~
+        code
+        ~~~~~
+        ''', '''
+        <pre><code>code
+        </code></pre>
+        ''');
+
+    validate('Pandoc style with language identifier', '''
+        ~~~~~dart
+        code
+        ~~~~~
+        ''', '''
+        <pre class="dart"><code>code
+        </code></pre>
+        ''');
+
+    validate('Pandoc style with inner tildes row', '''
+        ~~~~~
+        ~~~
+        code
+        ~~~
+        ~~~~~
+        ''', '''
+        <pre><code>~~~
+        code
+        ~~~
+        </code></pre>
+        ''');
   });
 
   group('Horizontal rules', () {