Parsing comments is a non-starter for all practical purposes.
They are free text, of zero or more characters, and all you can assume in FITS is US-ASCII.
; I would be quite within my rights to create this informative FITS header:
DATE-OBS= ‘2021-07-26T22:03:10’ /Ad ultimum diem et ad tempus.
// which tells you everything you need to know.
% I am with Nick on this one.
# Sorry, I couldn’t work out how to put a Perl or bash single-line comment into the subject as well.
<!–You have to make do with Fortran, Algol68, and C.–>