Use regex as key/car in alist

Question

I'd like to set up an alist (similar to auto-mode-alist), which is made up from regular expressions as key (AKA car) of the list. Next I'd like to be able, to match, say the major mode, against the regex in order to retrieve the value/cdr of the matching key/car.

(setq jb-string-mode-alist
     '(("\\(emacs-\\)?lisp\\(-interaction\\)?-mode" . "Combined")
        (lisp-interaction-mode . "Interaction") ; works
        (emacs-lisp-mode       . "Emacs")       ; works also
        (lisp-mode             . "Lisp")        ; works also
        (latex-mode            . "LaTeX")       ; works also 
        (".*$"                 . "Default")))

Afterwards I tried something like the following:

(assoc major-mode jb-tools-date-time-string-mode-alist) ; => "Interaction"

Evaluating the expression in the *scratch* buffer of emacs yields "Interaction" as the mode of the *scratch* buffer is lisp-interaction-mode. This matches the third entry in my alist, but it should match the first entry consisting of the regular expression.

Using assq instead of assoc makes no difference.

What is the trick, to evaluate the mode with respect to regular expressions?

Answer 1

assoc has the following documentation, which you can view by running C-h f assoc:

(assoc KEY ALIST &optional TESTFN)

Return non-nil if KEY is equal to the car of an element of ALIST.

The value is actually the first element of ALIST whose car equals KEY.

Equality is defined by the function TESTFN, defaulting to equal.
TESTFN is called with 2 arguments: a car of an alist element and KEY.

See that third argument? You can supply your own function there and it will be called instead of equal. If it returns true, the assoc be finished and will return the value associated with this key.

If you look in chapter 35.4 Regular Expression Searching of the Emacs Lisp manual, you will find that there is a function called string-match-p which returns t when the provided string matches the provided regexp. It is no coincidence that it takes the arguments in the same order that assoc provides them. Put the two together and call assoc like this:

(assoc major-mode jb-tools-date-time-string-mode-alist 'string-match-p)

Don’t forget that you can read the manuals inside of Emacs by running C-h i. Everything you need to know is in there.