The FRR builds in, at least, 1.4 are not linked against libpcre2, due to upstream not including that in the supplied debian/rules file (see: https://github.com/FRRouting/frr/issues/17365 )
For a standard 1.4:
% lddtree libfrr.so.0
libfrr.so.0 => ./libfrr.so.0 (interpreter => /lib64/ld-linux-x86-64.so.2)
libcap.so.2 => /usr/lib64/libcap.so.2 libyang.so.2 => not found libcrypt.so.1 => /usr/lib64/libcrypt.so.1 libm.so.6 => /lib64/libm.so.6 libprotobuf-c.so.1 => not found liblua5.3.so.0 => not found libjson-c.so.5 => /usr/lib64/libjson-c.so.5 libc.so.6 => /lib64/libc.so.6 ld-linux-x86-64.so.2 => /lib64/ld-linux-x86-64.so.2
for a patched build with libpcre2:
% lddtree libfrr.so.0
libfrr.so.0 => ./libfrr.so.0 (interpreter => /lib64/ld-linux-x86-64.so.2)
libcap.so.2 => /usr/lib64/libcap.so.2
libyang.so.2 => not found
libcrypt.so.1 => /usr/lib64/libcrypt.so.1
libm.so.6 => /lib64/libm.so.6
libprotobuf-c.so.1 => not found
libpcre2-posix.so.3 => /usr/lib64/libpcre2-posix.so.3
libpcre2-8.so.0 => /usr/lib64/libpcre2-8.so.0
liblua5.3.so.0 => not found
libjson-c.so.5 => /usr/lib64/libjson-c.so.5
libc.so.6 => /lib64/libc.so.6
ld-linux-x86-64.so.2 => /lib64/ld-linux-x86-64.so.2For a multi-fulltable border router, libpcre2 linking reduces the convergence time by 5-10 minutes.
As such, FRR in VyOS should be built against libpcre2.