diff --git a/include/class.format.php b/include/class.format.php
index 5cbcd048e9a26527e97311790a653a18b1855952..c8980326d08c7b404e8511942f35aebd0d79204c 100644
--- a/include/class.format.php
+++ b/include/class.format.php
@@ -383,7 +383,7 @@ class Format {
 
 
     function viewableImages($html, $script='image.php') {
-        return preg_replace_callback('/"cid:([\w.-_]{32})"/',
+        return preg_replace_callback('/"cid:([\w._-]{32})"/',
         function($match) use ($script) {
             $hash = $match[1];
             if (!($file = AttachmentFile::lookup($hash)))