I'm a big fan of ack, I love the --output feature, which is super handy. Unfortunately it is hardly ever installed when I need it. It's considerably slower than the highly tuned GNU regex code, which only really matters when you're grepping through a ton of material.

Could you give an example of something useful one could do with ack's --output feature?

The manual's a little light on the details.

All it says is:

  --output=expr
     Output the evaluation of expr for each line

What expression are they talking about? A regular expression? If so, how is that different from a normal grep?

A Perl expression. You could use this for sed-like activities:

    # Print included files.
    ack --output '$1' '#include <(.*)>'

You could presumably do much trickier things as well, since the --output argument can contain any Perl expression.

That's a great example. I just replaced it as the example in the Top 10 List on the front of betterthangrep.com. Thanks.

On Ubuntu, the package name is ack-grep. So use: sudo -i aptitude install ack-grep

Don't go for ack, which is some Kanji character converter.