Optimize the _RE_HR regexp to avoid catastrophic backtracking.

Also update the _RE_CODE regexp to allow leading space and the _RE_HEADER regexp to require a space-character after the leading `#`. The latter avoids matching long sequences of `#` which could also potentially lead to catastrophic backtracking.

pkgs/markdown/lib/src/block_parser.dart

diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index ce92867..6a390dd 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -9,13 +9,16 @@
 import 'util.dart';
 
 /// The line contains only whitespace or is empty.
-final _RE_EMPTY = new RegExp(r'^([ \t]*)$');
+final _RE_EMPTY = new RegExp(r'^(?:[ \t]*)$');
 
 /// A series of `=` or `-` (on the next line) define setext-style headers.
-final _RE_SETEXT = new RegExp(r'^((=+)|(-+))$');
+final _RE_SETEXT = new RegExp(r'^(=+|-+)$');
 
 /// Leading (and trailing) `#` define atx-style headers.
-final _RE_HEADER = new RegExp(r'^(#{1,6})(.*?)#*$');
+///
+/// Stats with 1-6 unescaped `#` characters which must not be followed by a
+/// non-space character. Line may end with any number of `#` characters,.
+final _RE_HEADER = new RegExp(r'^(#{1,6})[ \x09\x0b\x0c](.*?)#*$');
 
 /// The line starts with `>` with one optional space after.
 final _RE_BLOCKQUOTE = new RegExp(r'^[ ]{0,3}>[ ]?(.*)$');
@@ -24,14 +27,12 @@
 final _RE_INDENT = new RegExp(r'^(?:    |\t)(.*)$');
 
 /// Fenced code block.
-final _RE_CODE = new RegExp(r'^(`{3,}|~{3,})(.*)$');
+final _RE_CODE = new RegExp(r'^[ ]{0,3}(`{3,}|~{3,})(.*)$');
 
 /// 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.
-final _RE_HR = new RegExp(r'^[ ]{0,3}((-+[ ]{0,2}){3,}|'
-    r'(_+[ ]{0,2}){3,}|'
-    r'(\*+[ ]{0,2}){3,})$');
+final _RE_HR = new RegExp(r'^ {0,3}([-*_]) *\1 *\1(?:\1| )*$');
 
 /// Really hacky way to detect block-level embedded HTML. Just looks for
 /// "<somename".

pkgs/markdown/test/markdown_test.dart

diff --git a/pkgs/markdown/test/markdown_test.dart b/pkgs/markdown/test/markdown_test.dart
index 8368b84..b41d4b2 100644
--- a/pkgs/markdown/test/markdown_test.dart
+++ b/pkgs/markdown/test/markdown_test.dart
@@ -169,6 +169,18 @@
         <h6>header</h6>
         ''');
 
+    validate('h7 is not a header', '''
+        ####### header
+        ''', '''
+        <p>####### header</p>
+        ''');
+
+    validate('h6 must not be followed by non-space ', '''
+        ######A header
+        ''', '''
+        <p>######A header</p>
+        ''');
+
     validate('trailing "#" are removed', '''
         # header ######
         ''', '''