On 08/03/2017 06:28 AM, Santiago R.R. wrote:
> the -o option, which is supposed to return only the matching
> parts of the search, fails:

It's not failing. It's behaving as documented: -o outputs only nonempty 
matches. Otherwise, commands like 'grep -o "a*"' would output a separate 
line for each byte in the input. Although this behavior for -o is 
longstanding and is documented in the manual, it's not in the grep 
--help output so that's an oversight. I installed the attached to fix 
grep --help, and am closing the bug report on the GNU side.

Users who want to match empty lines can use 'grep "^$"', which is what 
I'd expect them to do anyway (-o would be superfluous there even if it 
included empty matches).