How to specify an ignore pattern with ack?

Question

I'm trying to search for lines containing 'foo' but these lines must not contain 'bar'. What's a good way to search with these parameters with ack?

I can do:

ack "foo" | grep -v "bar"

but it doesn't keep the nicely layed-out and coloured output of ack. Also, grep doesn't have good support for Perl regexes, unlike ack.

Answer 1

You can use a slightly more complicated regular expression:

ack '^(?!.*bar).*(foo)' file

The (?!...) does a negative lookahead to ensure that "bar" is not on the line. Here's how it works:

• ^ Start at beginning of the line.
• (?!.*bar) Advance any number of characters until "bar" is found. If it is found, do not continue. Otherwise, return to the start of the line and continue.
• .*(foo) Match (and capture) "foo" at any point between the beginning and end of the line.

Testing it out:

$ cat file
other foo bar
baz foo other
$ ack '^(?!.*bar).*(foo)' file
baz foo other