These thoughts lead me to suggest that maybe org-log-note-headings is no
longer sufficient to its original purpose, because extensions wish to
parse state changes, but that blocks users from configuring the formats.
Perhaps it is time to replace it with something that guarantees ability
to parse.
I have been experimenting with parsing the format-string to build a
regexp to match the generated-strings. This approach depends upon the
parsability of the expansions of each of the percent-codes. As concerns
org-log-note-headings, %t, %T, %d, %D, %s, %S, and %u all have parseable
expansions, as far as I can tell. I'm not sure about %U, but if the
rest of the string is not too complicated, it shouldn't be a problem.
Format-code flag, width, and precision add some complexity to the
problem of generating regexps to match the codes, and I haven't done
that bit yet. Overall I think this could be a very viable approach, and
I'll paste my scraps of code below:
;;; parsing state changes
;;;