$ perl -E'say "xxx" =~ s/(*MARK:a)^.|(*MARK:c).$|(*MARK:b)./$REGMARK/gr'
abc

##</code><code>##

$ python -c 'import pcre2; print(pcre2.sub("(*MARK:a)^.|(*MARK:c).$|(*MARK:b).", "$*MARK", "xxx"))'
abc