816-opt in the develop branch uses regex.h from libtre which is no longer maintained.
I suggest moving to PCRE2 which provides a drop-in replacement with pcre2posix.h and libpcre2-posix.a.
Note, however, that PCRE2 requires PCRE2_STATIC to be defined when linking statically.
I suggest defining this in the Makefile if Windows was detected as target and include pcre2posix.h instead of regex.h in helpers.h if PCRE2_STATIC was defined.