Speed up markdown parsing with fast path for plain words.

pkg/dartdoc/lib/src/markdown/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.
 
 /// Maintains the internal state needed to parse inline span elements in
 /// markdown.
 class InlineParser {
   static List<InlineSyntax> get syntaxes {
     // Lazy initialize.
     if (_syntaxes == null) {
       _syntaxes = <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.
+        new TextSyntax(r'\s*[A-Za-z0-9]+'),
+
+        // The real syntaxes.
+
         new AutolinkSyntax(),
         new LinkSyntax(),
         // "*" 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 "&".
@@ skipping 365 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;
   }
 }