Findutils has a bug report about non-capturing ("shy") groups \(? in Emacs-dialect regular expressions ( https://savannah.gnu.org/bugs/index.php?65770).
>From the Emacs Texinfo manual: ‘\(?: ... \)’ specifies a “shy group” that does not record the matched substring; you can’t refer back to it with ‘\D’ (see below). This is useful in mechanically combining regular expressions, so that you can add groups for syntactic purposes without interfering with the numbering of the groups that are meant to be referred to. Is this a known issue? If yes, was there a decision not to implement this in gnulib, or is it a to-do? Thanks, James.