ack-grep: please help with 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?

Answer 1

ack "<\?="

? is a regex operator, so it needs escaping

Answer 2

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

Answer 3

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