bug#71252: why does grep match literal newlines when there are none, even with -z?

[Philippe Cerfon]

Hey.

I always thought, that grep is line based in a way that the current
string doesn't hold the line terminator.
If so, why does, e.g.:
  $ printf 'foo' | grep $'\n'
  foo
match?

Even with -z.
While:
  $ printf 'foo\nbar' | grep -z $'\n'
  foo
  bar
would make sense to me, why does it also match:
  $ printf 'foobar' | grep -z $'\n'
  foobar
?


In PCRE mode:
  $ printf 'foobar' | grep -P -z '\n'
  $
No match, that I would expect.
  $ printf 'foo\nbar' | grep -P -z '\n'
  foo
  bar
Match, again, expected.
But:
  $ printf 'foobar' | grep -P -z $'\n'
  foobar
Why does that match?

Thanks,
Philippe