Avoid optional groups in tuareg-other-file-alist

`ff-find-the-other-file' does not behave well in presence of an
optional group at the "start".

Fixes #280

CHANGES.md

@@ -26,7 +26,7 @@ Backward incompatible changes are marked with “⚠”.
   levels of fontification (see the README).
 * The switch .ml ↔ .mli now uses the Emacs built-in `find-file` and
   was extended to `.eliom` ↔ `.eliomi` and `.mly` ↔ `.mli`.  It also
-  work for pre-processed files named `.pp.ml` and `.pp.mli`.
+  works for pre-processed files named `.pp.ml` and `.pp.mli`.
 * When switching from an `.ml` to a non-existing `.mli` file using
   <kbd>C-c C-a</kbd>, one is offered to fill the `.mli` buffer with the
   generated interface.

tuareg.el

@@ -219,8 +219,10 @@ much less efficient."
   :group 'tuareg :type 'boolean)
 
 (defcustom tuareg-other-file-alist
-  '(("\\.\\(?:pp\\.\\)?mli\\'" (".ml" ".mll" ".mly" ".pp.ml"))
-    ("\\.\\(?:pp\\.\\)?ml\\'" (".mli"))
+  '(("\\.pp\\.mli\\'" (".ml" ".mll" ".mly" ".pp.ml"))
+    ("\\.mli\\'" (".ml" ".mll" ".mly" ".pp.ml"))
+    ("\\.pp\\.ml\\'" (".mli"))
+    ("\\.ml\\'" (".mli"))
     ("\\.mll\\'" (".mli"))
     ("\\.mly\\'" (".mli"))
     ("\\.eliomi\\'" (".eliom"))