bug#21414: -F string with tailing newline always matches

[Paul Eggert]

Jim Meyering wrote:
> I too find this behavior surprising:
>
> $ seq 3|grep -F xxx$'\n'
> 1
> 2
> 3
>
> This feels like a bug, since it's an artifact of how grep accumulates
> multiple keys internally: it uses newline as the separator
> (http://git.savannah.gnu.org/cgit/grep.git/tree/src/grep.c#n2308).
> Including a literal newline in the search string conflicts with that.

It's not an artifact; it's intended behavior.  POSIX says that xxx$'\n' (which 
expands to three 'x's followed by a newline) is a pattern_list, not a pattern. 
A pattern_list is defined to be a series of patterns separated by newlines (not 
terminated by newlines), so that pattern_list has two patterns, xxx and the 
empty pattern.