diff --git a/Parsedown.php b/Parsedown.php
index 4e07a15..fad7739 100755
--- a/Parsedown.php
+++ b/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);
 		}
 
diff --git a/tests/data/emphasis_-_em_strong.html b/tests/data/emphasis_-_em_strong.html
new file mode 100644
index 0000000..6d8da9c
--- /dev/null
+++ b/tests/data/emphasis_-_em_strong.html
@@ -0,0 +1,5 @@
+<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
diff --git a/tests/data/emphasis_-_em_strong.md b/tests/data/emphasis_-_em_strong.md
new file mode 100644
index 0000000..32effdc
--- /dev/null
+++ b/tests/data/emphasis_-_em_strong.md
@@ -0,0 +1,9 @@
+___em strong___
+
+___one_ at the start__
+
+__one at the _end___
+
+__one _in the_ middle__
+
+**one with *asterisks***
\ No newline at end of file