I would prefer the goal of 'sed --posix' to be disabling only GNU extensions
that conflict with POSIX, rather than crippling sane semantics for
sequences intentionally left undefined by POSIX.
The purpose that you desire is available through the POSIXLY_CORRECT=1
environment setting. I'm talking about --posix; this is a different thing.
I, for one, wish I had noted earlier that some sed expressions that I was
using were not portable according to POSIX. --posix appears to be made for
this purpose. Like -ansi -pedantic for gcc.