I have two scripts which I would like to run both in a Linux machine and in a MacOS machine. But a different behaviour of the egrep command makes those scripts generate different outputs.
In particular, this is what happens when I use egrep on Linux (Ubuntu):
$ echo ".test" | egrep "[a-zA-Z0-9]*"
.test
$ echo ".test" | egrep -o "[a-zA-Z0-9]*"
test
$
and this is what happens when I use egrep on MacOS
$ echo ".test" | egrep "[a-zA-Z0-9]*"
.test
$ echo ".test" | egrep -o "[a-zA-Z0-9]*"
$
The first behaviour is what I would expect, the second one (empty output) is unexpected. Perhaps this is a bug in the implementation of egrep with -o option under MacOS?
Or, if the second behaviour is correct as well, do you know a way to obtain the same behaviour for the second case?
I tried to look at the corresponding man pages for the two commands, this is extracted from the Linux man page:
-o, --only-matching
Print only the matched (non-empty) parts of a matching line, with each
such part on a separate output line.
and this is extracted from the man page for MacOS:
-o, --only-matching
Prints only the matching part of the lines.
Although the descriptions seem a bit different, the meaning of the two options seems to be the same, so why is egrep -o behaving differently in MacOS? Am I not considering any subtle aspect of this command?
This depends on how the different grep implementations deal with empty matches ([a-zA-Z0-9]* matches the empty string).
I wrote a longer text about this over at Unix&Linux.
In short, should all empty matches be returned? There are infinitely many such matches.