4.8 Custom Format Strings

Sometimes it is useful to allow users and Lisp programs alike to control how certain text is generated via custom format control strings. For example, a format string could control how to display someone’s forename, surname, and email address. Using the function format described in the previous section, the format string could be something like "%s %s <%s>". This approach quickly becomes impractical, however, as it can be unclear which specification character corresponds to which piece of information.

A more convenient format string for such cases would be something like "%f %l <%e>", where each specification character carries more semantic information and can easily be rearranged relative to other specification characters, making such format strings more easily customizable by the user.

The function format-spec described in this section performs a similar function to format, except it operates on format control strings that use arbitrary specification characters.

Function: format-spec template spec-alist &optional ignore-missing split

This function returns a string produced from the format string template according to conversions specified in spec-alist, which is an alist (see Association Lists) of the form (letter . replacement). Each specification %letter in template will be replaced by replacement when formatting the resulting string.

The characters in template, other than the format specifications, are copied directly into the output, including their text properties, if any. Any text properties of the format specifications are copied to their replacements.

Using an alist to specify conversions gives rise to some useful properties:

  • If spec-alist contains more unique letter keys than there are unique specification characters in template, the unused keys are simply ignored.
  • If spec-alist contains more than one association with the same letter, the closest one to the start of the list is used.
  • If template contains the same specification character more than once, then the same replacement found in spec-alist is used as a basis for all of that character’s substitutions.
  • The order of specifications in template need not correspond to the order of associations in spec-alist.

REPLACEMENT can also be a function taking no arguments, and returning a string to be used for the replacement. It will only be called when the corresponding LETTER is used in the TEMPLATE. This is useful, for example, to avoid prompting for input unless it is needed.

The optional argument ignore-missing indicates how to handle specification characters in template that are not found in spec-alist. If it is nil or omitted, the function signals an error; if it is ignore, those format specifications are left verbatim in the output, including their text properties, if any; if it is delete, those format specifications are removed from the output; any other non-nil value is handled like ignore, but any occurrences of ‘%%’ are also left verbatim in the output.

If the optional argument split is non-nil, instead of returning a single string, format-spec will split the result into a list of strings, based on where the substitutions were performed. For instance:

(format-spec "foo %b bar" '((?b . "zot")) nil t)
     ⇒ ("foo " "zot" " bar")

The syntax of format specifications accepted by format-spec is similar, but not identical, to that accepted by format. In both cases, a format specification is a sequence of characters beginning with ‘%’ and ending with an alphabetic letter such as ‘s’.

Unlike format, which assigns specific meanings to a fixed set of specification characters, format-spec accepts arbitrary specification characters and treats them all equally. For example:

(setq my-site-info
      (list (cons ?s system-name)
            (cons ?t (symbol-name system-type))
            (cons ?c system-configuration)
            (cons ?v emacs-version)
            (cons ?e invocation-name)
            (cons ?p (number-to-string (emacs-pid)))
            (cons ?a user-mail-address)
            (cons ?n user-full-name)))

(format-spec "%e %v (%c)" my-site-info)
     ⇒ "emacs 27.1 (x86_64-pc-linux-gnu)"

(format-spec "%n <%a>" my-site-info)
     ⇒ "Emacs Developers <emacs-devel@gnu.org>"

A format specification can include any number of the following flag characters immediately after the ‘%’ to modify aspects of the substitution.


This flag causes any padding specified by the width to consist of ‘0’ characters instead of spaces.


This flag causes any padding specified by the width to be inserted on the right rather than the left.


This flag causes the substitution to be truncated on the left to the given width and precision, if specified.


This flag causes the substitution to be truncated on the right to the given width, if specified.


This flag converts the substituted text to upper case (see Case Conversion in Lisp).

_ (underscore)

This flag converts the substituted text to lower case (see Case Conversion in Lisp).

The result of using contradictory flags (for instance, both upper and lower case) is undefined.

As is the case with format, a format specification can include a width, which is a decimal number that appears after any flags, and a precision, which is a decimal-point ‘.’ followed by a decimal number that appears after any flags and width.

If a substitution contains fewer characters than its specified width, it is padded on the left:

(format-spec "%8a is padded on the left with spaces"
             '((?a . "alpha")))
     ⇒ "   alpha is padded on the left with spaces"

If a substitution contains more characters than its specified precision, it is truncated on the right:

(format-spec "%.2a is truncated on the right"
             '((?a . "alpha")))
     ⇒ "al is truncated on the right"

Here is a more complicated example that combines several aforementioned features:

(setq my-battery-info
      (list (cons ?p "73")      ; Percentage
            (cons ?L "Battery") ; Status
            (cons ?t "2:23")    ; Remaining time
            (cons ?c "24330")   ; Capacity
            (cons ?r "10.6")))  ; Rate of discharge

(format-spec "%>^-3L : %3p%% (%05t left)" my-battery-info)
     ⇒ "BAT :  73% (02:23 left)"

(format-spec "%>^-3L : %3p%% (%05t left)"
             (cons (cons ?L "AC")
     ⇒ "AC  :  73% (02:23 left)"

As the examples in this section illustrate, format-spec is often used for selectively formatting an assortment of different pieces of information. This is useful in programs that provide user-customizable format strings, as the user can choose to format with a regular syntax and in any desired order only a subset of the information that the program makes available.