diff --git a/wagtail/wagtailadmin/static_src/wagtailadmin/js/page-editor.js b/wagtail/wagtailadmin/static_src/wagtailadmin/js/page-editor.js
index 3e9c2d86b..552260b0b 100644
--- a/wagtail/wagtailadmin/static_src/wagtailadmin/js/page-editor.js
+++ b/wagtail/wagtailadmin/static_src/wagtailadmin/js/page-editor.js
@@ -282,7 +282,7 @@ function cleanForSlug(val, useURLify) {
     if (URLify != undefined && useURLify !== false) { // Check to be sure that URLify function exists, and that we want to use it.
         return URLify(val, val.length);
     } else { // If not just do the "replace"
-        return val.replace(/\s/g, '-').replace(/[^A-Za-z0-9\-]/g, '').toLowerCase();
+        return val.replace(/\s/g, '-').replace(/[^A-Za-z0-9\-\_]/g, '').toLowerCase();
     }
 }