The format-time-string function takes a string, and replaces a set of special constructs in that string (characters preceeded by %) with some specific text.

I'd like to implement a functionality like that in a function of my own:

  • I have an alist between chars and symbols like: '((?n . name) (?r . reputation)).
  • The function should take a string like "My name is %n, and my rep is %r".
  • It should replace %n and %r with the value of the variables name and reputation, and return the result.
  • It should handle %% just like format-time-string (replace it with %).

What's the easiest way to implement this function?
Is there a library or function that will facilitate this? Handling %% correctly is important.


3 Answers 3


The default functions to be used are format-spec and format-spec-make:

(let* ((name "Malabarba")
       (reputation "good")
       (fs (format-spec-make ?n name ?r reputation)))
  (format-spec "My name is %n, with a %r reputation.  %% is kept." fs))
  • 1
    Nice, didn't expect such a thing to exist in Emacs already. What's particularly good is that there's rudimentary error handling for invalid format codes included.
    – wasamasa
    Commented Jan 7, 2015 at 12:34

Since elisp has rather primitive facilities for dealing with strings, it is better to put your format string into a buffer and iterate over that:

(defvar malabarba-alist '((?n . "Malabarba") (?r . 7488) (?% . "%")))

(defun malabarba-format (string)
    (insert string)
    (goto-char 1)
    (while (search-forward "%" nil t)
      (let ((s (cdr (assoc (char-after) malabarba-alist))))
          (s (delete-char -1) (delete-char 1) (insert (format "%s" (eval s))))
          (t (unless (eobp) (forward-char))))))

A minor subtlety — this doesn't break when there's a lone % at the end of the string because char-after returns nil at the end of the buffer.

  • Yes, this approach, of progressing through the string once, used also by @AlanShutko, makes more sense. I've deleted my answer.
    – Drew
    Commented Jan 7, 2015 at 7:20

Here is code that I use for a format-time-string for a fake calendar:

(defun mystcal-format-time (format-string time)
  "Format time
%Y is the year.
%m is the numeric month.
%B is the full name of the month.
%d is the day of the month, zero-padded, %e is blank-padded.
%u is the numeric day of week from 1 (Monday) to 7, %w from 0 (Sunday) to 6.
%A is the locale's full name of the day of week.
%H is the hour on a 24-hour clock, %I is on a 12-hour clock, %k is like %H
 only blank-padded, %l is like %I blank-padded.
%p is the locale's equivalent of either AM or PM.
%M is the minute.
%S is the second."
  (let* (output
         (start 0)
         (decoded-time (if (listp time)
                         (mystcal-decode-time time)))
         (turn (nth 0 decoded-time))
         (minute (nth 1 decoded-time))
         (hour (nth 2 decoded-time))
         (day (nth 3 decoded-time))
         (month (mystcal-month decoded-time))
         (year (nth 5 decoded-time))
         (weekday (mystcal-weekday-of-day day))
         (hour12 (mod hour 12)))
      (while (string-match "%" format-string start)
        (let ((index (match-beginning 0)))
          ;; First copy non-format text
          (setq output (concat output (substring format-string start 
          ;; Process format codes here
          (let (fmted)
            (setq output (concat output 
                                 (case (aref format-string (1+ index))
                                   (?Y (number-to-string year))
                                   (?m (number-to-string month))
                                   (?B (mystcal-month-name month))
                                   (?d (format "%02d" day))
                                   (?e (format "%2d" day))
                                   (?u (number-to-string (if (zerop weekday)
                                   (?w (number-to-string weekday))
                                   (?A (mystcal-weekday-name 
                                        (mystcal-weekday-of-day day)))
                                   (?H (format "%02d" hour))
                                   (?k (format "%2d" hour))
                                   (?I (format "%02d" (if (zerop hour12) 12 hour12)))
                                   (?l (format "%2d" (if (zerop hour12) 12 hour12)))
                                   (?p (if (< hour 12)
                                           "AM" "PM"))
                                   (?M (format "%02d" minute))
                                   (?S "00")))))
          (setq start (+ 2 index)))))
    (setq output (concat output (substring format-string start)))

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