Where are the man pages for GNU sed?
Prior to GNU sed v3.02, there weren’t any. Until recently, man pages distributed with gsed were borrowed from old sources or from other compilations. None of them were “official.” Even the man and info pages distributed with gsed 3.02 are incomplete. For example, they omit special regexes recognized by GNU sed not in most seds. See section 6.8.3 (“Special syntax in REs”), below.