Question

I am trying to pass arguments to the align-regexp function in Emacs (Lisp):

(defun align-on-comment-char ()
  (interactive)
  (align-regexp (region-beginning) (region-end) "#")
)

I would actually like to do this for all my modes specifically, where for each mode I want to bind an “align to comment character (; for emacs lisp, % for Latex, # for R)”.

What am I missing?

Answer 1

comment-start and comment-end are the usual variables to get the comment-string for the current mode.

You will also need to append some magic to get the matching right when
calling align-regexp. Like Oleg, I had to figure that out the hard way
by looking at the source. The error message here is not really
descriptive and I really consider this worthy of a bug-report or at
least a doc fix.

(defun align-comment (beg end)
  (interactive "r")
  (align-regexp beg end (concat "\\(\\s-*\\)" comment-start))
)