diff --git a/Parsedown.php b/Parsedown.php
index 4579e2f..7a84d06 100755
--- a/Parsedown.php
+++ b/Parsedown.php
@@ -35,9 +35,6 @@ class Parsedown
         $text = str_replace("\r\n", "\n", $text);
         $text = str_replace("\r", "\n", $text);
 
-        # replace tabs with spaces
-        $text = str_replace("\t", '    ', $text);
-
         # remove surrounding line breaks
         $text = trim($text, "\n");
 
@@ -145,6 +142,23 @@ class Parsedown
                 continue;
             }
 
+            if (strpos($line, "\t") !== false)
+            {
+                $parts = explode("\t", $line);
+
+                $line = $parts[0];
+
+                unset($parts[0]);
+
+                foreach ($parts as $part)
+                {
+                    $shortage = 4 - mb_strlen($line, 'utf-8') % 4;
+
+                    $line .= str_repeat(' ', $shortage);
+                    $line .= $part;
+                }
+            }
+
             $indent = 0;
 
             while (isset($line[$indent]) and $line[$indent] === ' ')
@@ -471,6 +485,11 @@ class Parsedown
                 $level ++;
             }
 
+            if ($level > 6 or $Line['text'][$level] !== ' ')
+            {
+                return;
+            }
+
             $text = trim($Line['text'], '# ');
 
             $Block = array(
@@ -614,7 +633,7 @@ class Parsedown
 
     protected function blockRule($Line)
     {
-        if (preg_match('/^(['.$Line['text'][0].'])([ ]{0,2}\1){2,}[ ]*$/', $Line['text']))
+        if (preg_match('/^(['.$Line['text'][0].'])([ ]*\1){2,}[ ]*$/', $Line['text']))
         {
             $Block = array(
                 'element' => array(
diff --git a/test/data/atx_heading.html b/test/data/atx_heading.html
index 3ab3fd5..751f873 100644
--- a/test/data/atx_heading.html
+++ b/test/data/atx_heading.html
@@ -4,6 +4,6 @@
 <h4>h4</h4>
 <h5>h5</h5>
 <h6>h6</h6>
-<h6>h6</h6>
+<p>####### not a heading</p>
 <h1>closed h1</h1>
 <p>#</p>
\ No newline at end of file
diff --git a/test/data/atx_heading.md b/test/data/atx_heading.md
index 5339a5b..ad97b44 100644
--- a/test/data/atx_heading.md
+++ b/test/data/atx_heading.md
@@ -10,7 +10,7 @@
 
 ###### h6
 
-####### h6
+####### not a heading
 
 # closed h1 #