grep/sed and some strange patterns/inputs

[Christoph Anton Mitterer]

Hey.

I've always had the impression that ^ and $ were the end/begin anchor
of the current pattern, and since e.g. grep/sed work normally in terms
of lines the start/end of lines.

What I found a bit strange is that e.g.:
printf '' | sed 's/^/foo/'
printf '' | sed 's/$/foo/'
printf '' | sed 's/^$/foo/'
doesn't produce foo and that e.g.
printf '' | grep '^'
printf '' | grep '$'
printf '' | grep '^$'
printf '' | grep '*'
don't match.

Why? Or better said, which part of POSIX mandates this? Or is it simply
"no stdin, nothing happens"?
OTOH, other tools do operate on that:
$ printf ''  | wc
      0       0       0


What looks IMO also a bit strange:
$ printf 'f' | sed "s/$/foo/"
foof
$printf 'f' | sed "s/$/foo/"
ffoo
(with no newlines)
So they do match,
even though there is no \n character.
OTOH
$ printf 'f\n' | sed
"s/$/foo/"
foof
$printf 'f\n' | sed "s/$/foo/"
ffoo
give the same, so while
there clearly is a \n now (and thus I'd assume a
new line) it's not
matched.
However:
$ printf 'f\n\n' | sed "s/^/foo/"
foof
foo
here it
matches,... so it seems it would just ignore trailing non-\n-ended
and
empty lines, but NOT trailing (or only) non-\n-ended but non-empty
lines
.
Seems a bit inconsequential.


Any ideas? Thanks,
Chris.

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