resolve #27

Parsedown.php

@@ -678,13 +678,13 @@ class Parsedown
 
 		if (strpos($text, '_') !== FALSE)
 		{
-			$text = preg_replace('/__(?=\S)(.+?)(?<=\S)__/s', '<strong>$1</strong>', $text);
+			$text = preg_replace('/__(?=\S)(.+?)(?<=\S)__(?!_)/s', '<strong>$1</strong>', $text);
 			$text = preg_replace('/_(?=\S)(.+?)(?<=\S)_/s', '<em>$1</em>', $text);
 		}
 
 		if (strpos($text, '*') !== FALSE)
 		{
-			$text = preg_replace('/\*\*(?=\S)(.+?)(?<=\S)\*\*/s', '<strong>$1</strong>', $text);
+			$text = preg_replace('/\*\*(?=\S)(.+?)(?<=\S)\*\*(?!\*)/s', '<strong>$1</strong>', $text);
 			$text = preg_replace('/\*(?=\S)(.+?)(?<=\S)\*/s', '<em>$1</em>', $text);
 		}
 

tests/data/emphasis_-_em_strong.html

+<p><strong><em>em strong</em></strong></p>
+<p><strong><em>one</em> at the start</strong></p>
+<p><strong>one at the <em>end</em></strong></p>
+<p><strong>one <em>in the</em> middle</strong></p>
+<p><strong>one with <em>asterisks</em></strong></p>
\ No newline at end of file

tests/data/emphasis_-_em_strong.md

+___em strong___
+
+___one_ at the start__
+
+__one at the _end___
+
+__one _in the_ middle__
+
+**one with *asterisks***
\ No newline at end of file