Several man pages refer to POSIX "regexp", but there is no definition of that easily findable, if it exists. The man page for "regexp" describes a program by that name and doesn't mention POSIX. See "See Also" section of "sed" man page for note of lack of such info. "regex" man page (which supposedly is POSIX) is no help either. "grep" and "slocate" are others.
assigned to teg
The man page is called regex(7), which is present and seems to be the one referenced.