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?

DVK · Accepted Answer · 2010-06-19T03:22:20.747

29

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

edited Jun 19 '10 at 03:22

answered Jun 19 '10 at 02:52

DVK

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Or use `\Q=\E` to escape ALL special characters. – Josh Kelley Jun 19 '10 at 03:06
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Or use the -Q option to escape all special characters, too, because really, who wants to mess with the \Q \E crap? – Andy Lester Jun 19 '10 at 04:51

score 20 · Answer 2 · answered Jun 19 '10 at 04:51

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

answered Jun 19 '10 at 04:51

Andy Lester

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score 15 · Answer 3 · answered Jan 30 '15 at 02:52

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ack -Q "<?="

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

(if you need not find by regular expression.)

answered Jan 30 '15 at 02:52

Junichiro Tobe

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score 8 · Answer 4 · answered Jun 19 '10 at 02:51

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ack "<\?="

? is a regex operator, so it needs escaping

answered Jun 19 '10 at 02:51

Jamie Wong

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ack-grep: chars escaping

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