diff --git a/lisp/files.el b/lisp/files.el
index d3c8629759d..279431ace4a 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -174,7 +174,7 @@ If the buffer is visiting a new file, the value is nil.")
   (cond ((and (eq system-type 'ms-dos) (not (msdos-long-file-names)))
 	 (concat "^\\([^A-Z[-`a-z]\\|..+\\)?:\\|" ; colon except after drive
 		 "[+, ;=|<>\"?*]\\|\\[\\|\\]\\|"  ; invalid characters
-		 "[\000-\031]|"			  ; control characters
+		 "[\000-\031]\\|"		  ; control characters
 		 "\\(/\\.\\.?[^/]\\)\\|"	  ; leading dots
 		 "\\(/[^/.]+\\.[^/.]*\\.\\)"))	  ; more than a single dot
 	((memq system-type '(ms-dos windows-nt))