I'm looking for a way in emacs to format a code like this:

// param1, param2, param3, param4
foo(9, 8, "bar", 3)

to this:

//  param1, param2, param3, param4
foo(9,      8,      "bar",  3)

I guess it should be feasible with align-regexp but I have troubles to express the regexp to use in this case (actually I don't really understand what the regexp is supposed to match). Any idea?

1 Answer 1


With indent-tabs-mode set to nil1, the following will do most of that:

C-uM-x align-regexp RET ,\(\s-*\) RET RET y

Dealing with the initial parameter is decidedly trickier. If you can guarantee the presence of a space after each comma, then you could use the following regexp along with 0 for the "Amount of spacing" question:

\(?:, \|^\(?://\|.*?(\)\)\(\s-*\)

You may prefer to use a two-step process, though. e.g.:

  • ^\(?://\|.*?(\)\(\s-*\) with spacing = 0, and not repeating; followed by
  • ,\(\s-*\) with spacing = 1, and repeating

You should probably elaborate on the kind of workflow you're looking for, though. Are you looking for a command to call manually? Automated alignment rules? Something to use occasionally via a keyboard macro? Other?

1 I would recommend using the following in general:

;; Align with spaces only
(defadvice align-regexp (around align-regexp-with-spaces)
  "Never use tabs for alignment."
  (let ((indent-tabs-mode nil))
(ad-activate 'align-regexp)
  • Thanks for your answer @phils. Actually I usually need this in a very specific case where I want to initialize some large chunk of data in unit tests with a function that takes many parameters: having comments and parameters aligned helps a lot for readability.
    – dlgd
    Jun 10, 2017 at 20:57

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