If it wasn't in single quotes, then that might be plausible, but I don't see it 
as the intent since no other aliases are excluded as possibilities for after 
the '/'. The initial "\n" makes 'n' the delimiter, the 2nd overrides it as 
being the BRE terminator, and the following 'n' is the terminator, before the 
'd' command. Should there be something explicit about aliases not being usable 
when repurposed as delimiter, maybe.
On Wednesday, March 25, 2020 Harald van Dijk <a...@gigawatt.nl> wrote:
On 25/03/2020 19:44, shwaresyst wrote:
> The GNU version is more correct, in my opinion, in that the use of n as 
> a delimiter should take precedence over its use as control character 
> alias with the wording as is. The other versions appear to consider the 
> BRE as <newline> so does not match 'n'.

You have that backwards, don't you? The GNU version lets the use of \n 
as a control character take precedence over its use as a delimiter. 
That's why n gets printed: \n\nn is treated as /\n/, which can never 
match any single-line string, so nothing gets deleted.

Likewise,

  echo n | sed '\n[^\n]nd'

prints nothing with GNU sed, but prints n with FreeBSD sed for the same 
reason: 'n' does contain a character that is not <newline>, but does not 
contain any character that is not <n>.

> ------------------------------------------------------------------------
> On Wednesday, March 25, 2020 Oğuz <oguzismailuy...@gmail.com> wrote:
> 
>      echo n | sed '\n\nnd'
> 
> Above command returns 'n' with GNU sed, and nothing with BSD seds and 
> OmniOS sed. [...]

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