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If you have a GPG-encrypted file, say notes.org.gpg, emacs figures out that the major mode has to be set to org-mode. This applies in general to any major mode out there; you do not need to modify the auto-mode-alist for each major mode to support that .gpg extension.

I would like to have my own extra extension, let's say .~bkp~ and have emacs ignore that when determining the major mode. Currently, if I have a file named notes.org.~bkp~, the org-mode is not set automatically.

I scoured through files.el, epa*.el, googled but couldn't figure out how emacs is ignoring the .gpg in the file names.

What function can I probably advice or a variable can I tweak to support my custom appended extensions?

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The relevant entry in auto-mode-alist is this:

 ([...] ("\\.gpg\\(~\\|\\.~[0-9]+~\\)?\\'" nil epa-file) [...])

And here is the documentation that describes what is going on:

auto-mode-alist is a variable defined in files.el.

This variable may be risky if used as a file-local variable.

Documentation: Alist of filename patterns vs corresponding major mode functions. Each element looks like (REGEXP . FUNCTION) or (REGEXP FUNCTION NON-NIL). (NON-NIL stands for anything that is not nil; the value does not matter.) Visiting a file whose name matches REGEXP specifies FUNCTION as the mode function to use. FUNCTION will be called, unless it is nil.

If the element has the form (REGEXP FUNCTION NON-NIL), then after calling FUNCTION (if it's not nil), we delete the suffix that matched REGEXP and search the list again for another match.

The extensions whose FUNCTION is archive-mode should also appear in auto-coding-alist with no-conversion coding system.

See also interpreter-mode-alist, which detects executable script modes based on the interpreters they specify to run, and magic-mode-alist, which determines modes based on file contents.

[...]

So there you go!

(add-to-list 'auto-mode-alist '("\\.bkp\\(~[0-9_]+~\\)\\'" nil backup-file))
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  • Thanks. I might be missing something here. Your explanation makes sense and I tried to implement it but it does not work: (add-to-list 'auto-mode-alist '("\\.bkp~[0-9_]+~\\'" nil backup-file)). It does not set the mode for file.sv.bkp~20150619_1427~ correctly. May be a second set of eyes of the above regexp can see the problem?
    – Kaushal Modi
    Jun 19, 2015 at 18:42
  • Even M-: (string-match "\\.bkp~[0-9_]+~\\'" (buffer-file-name)) (when I am in that file buffer) returns a non-nil value; but the major mode is still set to Fundamental mode. I also tried the same with a dummy notes.org.bkp~123~ file; same thing.
    – Kaushal Modi
    Jun 19, 2015 at 19:21
  • Interesting.. It works with other extensions (i have a few in my auto-mode-alist) but I just tried your specific example and get the same behavior. The file also gets opened read-only, which seems odd. Must be some other handling of that extension somewhere?
    – glucas
    Jun 19, 2015 at 19:31
  • @glucas I searched for bkp in the C-h v buffer for auto-mode-alist and the list I added is the only one in that alist.
    – Kaushal Modi
    Jun 19, 2015 at 19:41
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    Actually - what you are using is very close to what Emacs is already stripping off as version info. If I use a file name like foo.org.~bkp~1~ then Emacs visits that in org-mode without any additional rules in the auto-mode-alist. (Take a look at file-name-version-regexp.)
    – glucas
    Jun 19, 2015 at 19:59

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