How to match the car of alist elements with a regexp?

Question

I've spent a good amount of hours trying to solve this one but without success. I have an alist with strings

(setq trees '(("pine" . "cones") ("oak" . "acorns") ("maple" . "seeds")))

and I want to obtain the assoc of it using a regexp as key. That is

(assoc ".ine" trees)

always returns nil instead of ("pine" . "cones"). I've tried to use the assoc-default function instead, using string-match but it also did not work. I know that this is pretty basic, but I cannot really understand what I am doing wrong. Thank you in advance.

Answer 1

Pass a TESTFN arg to assoc. It needs to use string-match-p with the args reversed. That is, assoc passes TESTFN the pattern arg second, but string-match-p expects the pattern as its first arg.

(assoc ".*ine" trees (lambda (x regexp) (string-match-p regexp x)))

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I filed doc bug report #50110 for this, asking that the arg order for optional arg TESTFN be specified. And that bug has now been fixed!