Ack & negative lookahead giving errors

Question

I have a problem with using ack-grep with a negative look ahead.

I am running this command:

ack-grep "paypal_responded(?!_at)"

but I am getting the error:

bash: !_at: event not found

I have tried adding backslashes in various places, but I'm also new to using ack & linux, so please treat me as a newbie with any instructions.

Thanks in advance.

number5 · Accepted Answer · 2011-12-05T12:12:47.223

18

Try ack-grep 'paypal_responded(?!_at)'

You need single-quote to avoid bash interpret ! as history expand command.

edited Dec 05 '11 at 12:12

answered Dec 05 '11 at 12:06

number5

2

Great stuff. That's sorted it. Thanks :o) – TobyG Dec 05 '11 at 16:51
@TobyG Would you mind to accept this as an answer so that this question no longer remains unanswered? Thanks! – user1251007 Jan 09 '14 at 16:10
Sorry. This dates back to my early days on here didn't realise I'd not accepted an answer. Done now. – TobyG Jan 13 '14 at 09:15
This also works with zsh. It is way more practical than escaping all `( )` and `!` in regular expressions – Sébastien May 14 '14 at 09:27

sarnold · Answer 2 · 2011-12-06T00:31:17.997

4

The shell is interpreting the ! in your input as a command substitution:

$ ack-grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
$ !ac
ack-grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
$

You need to tell the shell that ! has no special meaning; there are two ways to do that:

ack-grep "paypal_responded(?\!_at)"

ack-grep "paypal_responded\(?\!_at\)"

or

ack-grep 'paypal_responded(?!_at)'

Single-quoted strings have fewer transformations applied to them:

$ ack-grep "s\!" /etc/passwd
$ ack-grep 's!' /etc/passwd
$

edited Dec 06 '11 at 00:31

answered Dec 05 '11 at 12:10

sarnold

I tried... ack-grep "paypal_responded(?\!_at)" .. but got the error... "ack-grep: Invalid regex 'paypal_responded(?\!_at)': Sequence (?\...) not recognized in regex; marked by <-- HERE in m/paypal_responded(?\ <-- HERE !_at)/" – TobyG Dec 05 '11 at 16:52
Sheesh, there's obviously more going on than I expected -- my simple examples worked exactly as I wanted, but the longer one with `(?!_at)` fails in horrible ways and it's easy to see why you were driving yourself nuts trying to make it work. `ack-grep "paypal_responded\(?\!_at\)"` works fine.... but I can't explain it. – sarnold Dec 06 '11 at 00:30

2 Answers2