On 12/14/24 01:33, Eli Zaretskii wrote:

I'm guessing this is because Emacs 27 switched to the Gnulib's regex
implementation in etags and other lib-src programs, whereas previous
versions used the Emacs's own regex code (which is still used for
Emacs's own regex search and replacement code).

CC'ing Paul Eggert, in the hope he can tell whether this is expected
or not, or how to fix it.

Yes that's expected, as glibc/Gnulib regex doesn't do shy groups. A workaround is to not use shy groups, e.g.:

etags --regex='/DEFINE_FF *(\(TAG \)? *\([^)]+\)/\2/' \
  --output ETAGS-test foo.c

This should be portable between both older and newer etags.