ack-grep: chars escaping

Question

My goal is to find all "<?=" occurrences with ack. How can I do that?

ack "<?="

Doesn't work. Please tell me how can I fix escaping here?

Calchas

Answer 1

Since ack uses Perl regular expressions, your problem stems from the fact that in Perl RegEx language, ? is a special character meaning "last match is optional". So what you are grepping for is = preceded by an optional <

So you need to escape the ? if that's just meant to be a regular character.

To escape, there are two approaches - either <\?= or <[?]=; some people find the second form of escaping (putting a special character into a character class) more readable than backslash-escape.

UPDATE As Josh Kelley graciously added in the comment, a third form of escaping is to use the \Q operator which escapes all the following special characters till \E is encountered, as follows: \Q<?=\E

Japan

Or use \Q<?=\E to escape ALL special characters. – Pallaton

Or use the -Q option to escape all special characters, too, because really, who wants to mess with the \Q \E crap? – Foumart

Answer 2

Rather than trying to remember which characters have to be escaped, you can use -Q to quote everything that needs to be quoted.

Foumart

Answer 3

ack -Q "<?="

This is the best solution if you will want to find by simple text.

(if you need not find by regular expression.)

Whopping

Answer 4

ack "<\?="

? is a regex operator, so it needs escaping

Garbanzo