diff --git a/lib/src/block_parser.dart b/lib/src/block_parser.dart
index 504fdbbe..5bdc1115 100644
--- a/lib/src/block_parser.dart
+++ b/lib/src/block_parser.dart
@@ -9,13 +9,16 @@ import 'document.dart';
 import 'util.dart';
 
 /// The line contains only whitespace or is empty.
-final _emptyPattern = new RegExp(r'^([ \t]*)$');
+final _emptyPattern = new RegExp(r'^(?:[ \t]*)$');
 
 /// A series of `=` or `-` (on the next line) define setext-style headers.
-final _setextPattern = new RegExp(r'^((=+)|(-+))$');
+final _setextPattern = new RegExp(r'^(=+|-+)$');
 
 /// Leading (and trailing) `#` define atx-style headers.
-final _headerPattern = new RegExp(r'^(#{1,6})(.*?)#*$');
+///
+/// Starts with 1-6 unescaped `#` characters which must not be followed by a
+/// non-space character. Line may end with any number of `#` characters,.
+final _headerPattern = new RegExp(r'^(#{1,6})[ \x09\x0b\x0c](.*?)#*$');
 
 /// The line starts with `>` with one optional space after.
 final _blockquotePattern = new RegExp(r'^[ ]{0,3}>[ ]?(.*)$');
@@ -24,14 +27,12 @@ final _blockquotePattern = new RegExp(r'^[ ]{0,3}>[ ]?(.*)$');
 final _indentPattern = new RegExp(r'^(?:    |\t)(.*)$');
 
 /// Fenced code block.
-final _codePattern = new RegExp(r'^(`{3,}|~{3,})(.*)$');
+final _codePattern = new RegExp(r'^[ ]{0,3}(`{3,}|~{3,})(.*)$');
 
 /// 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 _hrPattern = new RegExp(r'^[ ]{0,3}((-+[ ]{0,2}){3,}|'
-    r'(_+[ ]{0,2}){3,}|'
-    r'(\*+[ ]{0,2}){3,})$');
+final _hrPattern = new RegExp(r'^ {0,3}([-*_]) *\1 *\1(?:\1| )*$');
 
 /// Really hacky way to detect block-level embedded HTML. Just looks for
 /// "<somename".
diff --git a/test/markdown_test.dart b/test/markdown_test.dart
index 4facb3b3..b7472899 100644
--- a/test/markdown_test.dart
+++ b/test/markdown_test.dart
@@ -235,6 +235,24 @@ void main() {
         <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',
         '''