diff --git a/src/markup/engine/__tests__/remarkup/underline.txt b/src/markup/engine/__tests__/remarkup/underline.txt
index 71c50c5..511c0ce 100644
--- a/src/markup/engine/__tests__/remarkup/underline.txt
+++ b/src/markup/engine/__tests__/remarkup/underline.txt
@@ -1,11 +1,13 @@
 omg__ wtf_____ bbq___ lol__
 __underlined text__
 __This is a great idea___ die forever please
 __
+/__notunderlined__/ and also /__notunderlined__.c
 ~~~~~~~~~~
 <p>omg__ wtf_____ bbq___ lol__
 <u>underlined text</u>
 <u>This is a great idea_</u> die forever please
-__</p>
+__
+/__notunderlined__/ and also /__notunderlined__.c</p>
 ~~~~~~~~~~
-omg__ wtf_____ bbq___ lol__ __underlined text__ __This is a great idea___ die forever please __
+omg__ wtf_____ bbq___ lol__ __underlined text__ __This is a great idea___ die forever please __ /__notunderlined__/ and also /__notunderlined__.c
diff --git a/src/markup/engine/remarkup/markuprule/PhutilRemarkupRuleUnderline.php b/src/markup/engine/remarkup/markuprule/PhutilRemarkupRuleUnderline.php
index a389efa..9cb4137 100644
--- a/src/markup/engine/remarkup/markuprule/PhutilRemarkupRuleUnderline.php
+++ b/src/markup/engine/remarkup/markuprule/PhutilRemarkupRuleUnderline.php
@@ -1,25 +1,25 @@
 <?php
 
 final class PhutilRemarkupRuleUnderline
   extends PhutilRemarkupRule {
 
   public function getPriority() {
     return 1000.0;
   }
 
   public function apply($text) {
     if ($this->getEngine()->isTextMode()) {
       return $text;
     }
 
     return $this->replaceHTML(
-      '@(?<!_)__([^\s_].*?_*)__@s',
+      '@(?<!_|/)__([^\s_/].*?_*)__(?!/|\.\S)@s',
       array($this, 'applyCallback'),
       $text);
   }
 
   protected function applyCallback($matches) {
     return hsprintf('<u>%s</u>', $matches[1]);
   }
 
 }