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
来源：https://stackoverflow.com/questions/3074122/ack-grep-chars-escaping