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.

Gutter

Answer 1

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

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

Humes

Great stuff. That's sorted it. Thanks :o) – Gutter

@Gutter Would you mind to accept this as an answer so that this question no longer remains unanswered? Thanks! – Ingridingrim

Sorry. This dates back to my early days on here didn't realise I'd not accepted an answer. Done now. – Gutter

This also works with zsh. It is way more practical than escaping all ( ) and ! in regular expressions – Sinistrality

Answer 2

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
$ 

Tomato

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)/" – Gutter

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. – Tomato