Catch when regexp matches empty string

[epa]

When the regexp passed to ack matches the empty string, this is much more likely to be a mistake than intentional. Rather than print the complete contents of every file, ack should stop with a diagnostic.

% ack ''
The regular expression matches the empty string, so would find all lines.

(If there's really a use for it, then maybe an --allow-empty flag similar to what you have in git to allow empty commits. And of course, having detected that the regexp matches the empty string, ack could go a bit faster by not bothering to match it.)

(For example I mistyped (joe|jim)? when I had meant to search for (joe|jim)@. Just to give a random example.)

[petdance]

This makes a lot of sense. Let me ponder if we need an --allow-empty. I'm trying to imagine a case where you would want it. Maybe someone searches for .*whatever just so they get every line, even those that don't match whatever? Like they're emulating the --passthru option.

[epa]

Yes, I don't think --allow-empty would be needed in practice but I mentioned it because you sometimes get people who furiously defend the current behaviour, whatever it may be.

[petdance]

I don't optimize for complaint avoidance. If people complain and there's a legit need we can add it.