diff --git a/functions.php b/functions.php
index 9c62f3d..e10c7a4 100644
--- a/functions.php
+++ b/functions.php
@@ -372,7 +372,7 @@ add_action('save_post', 'save_article_meta_boxes');
 
 function add_doi_rewrite_rule()
 {
-    add_rewrite_rule('^([0-9]{2})\.([0-9]{4})/([a-zA-Z0-9_\-\/]+)?$', 'index.php?doi=$matches[1].$matches[2]/$matches[3]', 'top');
+    add_rewrite_rule('^([0-9]{2})\.([0-9]{4,})/([a-zA-Z0-9_\-\/]+)?$', 'index.php?doi=$matches[1].$matches[2]/$matches[3]', 'top');
 }
 
 add_action('init', 'add_doi_rewrite_rule');