diff --git a/lib/6to5/transformation/modules/_default.js b/lib/6to5/transformation/modules/_default.js
index 300c12ab18..1bc78c15ab 100644
--- a/lib/6to5/transformation/modules/_default.js
+++ b/lib/6to5/transformation/modules/_default.js
@@ -171,7 +171,7 @@ DefaultFormatter.prototype.getModuleName = function () {
 
   if (!opts.keepModuleIdExtensions) {
     // remove extension
-    filenameRelative = filenameRelative.replace(/\.(.*?)$/, "");
+    filenameRelative = filenameRelative.replace(/\.(\w*?)$/, "");
   }
 
   moduleName += filenameRelative;