I am looking for an alternative to file-name-extension that will include an ending ~ if there is one. For example, if there is a file named foo.el~, the result should be el~.
Rather than using the generic split-string, I was hoping there might be something more specific geared towards file extensions.
The built-in function file-name-extension uses file-name-sans-versions, which does us the favor of removing the tilde. http://www.gnu.org/software/emacs/manual/html_node/elisp/File-Name-Components.html Removing file-name-sans-versions from said function achieves the desired effect.
(defun my-file-name-extension (filename &optional period)
"This function is a modification of `file-name-extension`, in that the function
`file-name-sans-extensions` has been removed -- thus, the tilde (if any) remains."
(save-match-data
(let ((file (file-name-nondirectory filename)))
(if (and (string-match "\\.[^.]*\\'" file)
(not (eq 0 (match-beginning 0))))
(substring file (+ (match-beginning 0) (if period 0 1)))
(if period
"")))))