>> Yes.  You get, for each match: the line number (from the beginning of 
>> the file), the byte offset (from the beginning of the file) of the 
>> first displayed character, and the context of the match.
>
> OK, so we get the byte offset, but not the length of the match (which 
> we'll also need later, for purposes such as highlighting and 
> replacement). And what happens if there are several matches on the same 
> line? We need columns for all of them.
>

I don't know exactly what you want to do, I initially chimed in this 
conversation to react to Juri's "GNU grep has no option to truncate 
output", to mention that GNU grep does have an option to do this; perhaps 
it doesn't do exactly what you want.

I could be wrong, but I believe that adapting what you want to what GNU 
grep provides will always be more efficient than the opposite.

>> And you can easily get the byte offset of each beginning of line with 
>> "grep -nbo '^.'", so calculating the byte offset from the beginning of 
>> the line is easy.
>
> Do you mean to suggest we call grep one more time for each matching 
> line?
>

No, once for each file.  "grep -nbo '^.' FILE" returns a "<line>:<offset 
of first char>:<first char>" line for each line in FILE.  With this you 
can easily calculate the offset of a match on a given line.  This will be 
more efficient than calculating the offset of a match by parsing each line 
with Elisp code.