ack-grep: chars escaping

问题:

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?

回答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

回答2:

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

回答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.)

回答4:

ack "<\?="

? is a regex operator, so it needs escaping