it may be worth to have a peek into the posix document describing
the required standard behavior of sed. since most of us use linux
and thus GNU tools with their many extensions, we have a tendency
in getting sloppy about writing portable script code. we've seen
portability issues due to e.g. bash-isms before that broke scripts
when executed on ubuntu/debian since those don't use bash as /bin/sh.
In that same vein, GNU sed has an option "--posix" that disables all GNU
extensions. At least, that's what it SAYS it does. It's probably a good
idea to test all scripts that use sed with that option in place
(temporarily; leaving it in permanently is decidedly NOT portable) if
there is ever any doubt.
Many GNU/Linux distributions come, often optionally, with either ash or
dash, which are implementations of the Ackland shell (which is
sh-compatible but not Bash-compatible). Shell scripts could be tested
that way for portability.
Karl