Ignore underscores in the middle of words.

lib/src/inline_parser.dart

 // 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';
 
@@ skipping 25 matching lines @@
     // Leave already-encoded HTML entities alone. Ensures we don't turn
     // "&" into "&"
     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: '&lt;'),
     // Parse "**strong**" tags.
     new TagSyntax(r'\*\*', tag: 'strong'),
     // Parse "__strong__" tags.
-    new TagSyntax(r'__', tag: 'strong'),
+    new TagSyntax(r'\b__', tag: 'strong', end: r'__\b'),

[nweiz, 2015/09/14 21:52:13]

Using \b here isn't very non-English-friendly; it'll only take ASCII
alphanumeric characters into account
(http://www.ecma-international.org/ecma-262/5.1/#sec-15.10.2.6), so something
like á_ḇ_č will still get formatted wrong.

[Bob Nystrom, 2015/09/15 00:19:16]

Hmm, good point. I think this is sort of a stopgap solution until a more
comprehensive implementation that follows the full CommonMark spec can be done.

     // 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'),
+    new TagSyntax(r'\b_', tag: 'em', end: r'_\b'),
     new CodeSyntax(),
     // 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;
 
@@ skipping 411 matching lines @@
       parser.consume(endMatch[0].length);
     } else {
       // Didn't close correctly so revert to text.
       parser.start = startPos;
       parser.advanceBy(endMatch[0].length);
     }
 
     return null;
   }
 }