diff --git a/src/js/ublock.js b/src/js/ublock.js
index 82b43c285..d2a891eda 100644
--- a/src/js/ublock.js
+++ b/src/js/ublock.js
@@ -266,8 +266,9 @@ var matchBucket = function(url, hostname, bucket, start) {
     return whitelist;
 };
 
-µBlock.reWhitelistBadHostname = /[^a-z0-9.\-\[\]:]/;
-µBlock.reWhitelistHostnameExtractor = /([a-z0-9\[][a-z0-9.\-]*[a-z0-9\]])(?::[\d*]+)?\/(?:[^\x00-\x20\/]|$)[^\x00-\x20]*$/;
+// https://github.com/gorhill/uBlock/issues/3717
+µBlock.reWhitelistBadHostname = /[^a-z0-9.\-_\[\]:]/;
+µBlock.reWhitelistHostnameExtractor = /([a-z0-9.\-_\[\]]+)(?::[\d*]+)?\/(?:[^\x00-\x20\/]|$)[^\x00-\x20]*$/;
 
 /******************************************************************************/