When using the Emacs mode of regular expressions in gnulib, \(?: is not supported - but this is supported by regular expressions in Emacs itself.
This is useful to support because \(?: is frequently used by Emacs features which generate regular expressions (namely rx and regexp-opt) and supporting \(?: allows those regular expressions to be used in other GNU tools. See also my previous bug report in find here: https://savannah.gnu.org/bugs/?65770
