CODEWITHSUNDEEP
regexperl
Tiago Lopo
Tiago Lopo

Reputation: 7959

Can't understand perl regex to unquote meta

I came across this awesome regex:

s/((?:\\[a-zA-Z\\])+)/qq[qq[$1]]/eeg

It does magic, but is so obscure I can't understand it. It works very well:

echo 'a\tb\nc\r\n' | perl -lpe 's/((?:\\[a-zA-Z\\])+)/qq[qq[$1]]/eeg'
a   b
c

Let us watch it with cat -A :

echo 'a\tb\nc\r\n' | perl -lpe 's/((?:\\[a-zA-Z\\])+)/qq[qq[$1]]/eeg' | cat -A
a^Ib$
c^M$
$

I will keep it for future reference, but it would be really cool to understand it. I know /ee modifier evaluates RHS, but what are those qqs? Is the function qq for double quotes ? I would appreciate if someone could explain.

PS. I found this regex here

Upvotes: 0

Views: 464

Answers (2)

jcaron
jcaron

Reputation: 17710

qq is the interpolating quote operator. It's the same thing as putting a string between double quotes, but can use open-close character pairs like [] here. This has the advantage that you can nest it, which you couldn't do with double quotes.

Upvotes: 1

Rob11311
Rob11311

Reputation: 1416

In perl re's you have single and double quotes, where "$foo" is expanded and '$foo' is literal.

The q operator lets you set which character does ' The qqoperator sets the character for ".

So in the awesome example, [ is getting set to expand variables, and perl magic is making it more readable by pairing ] with [. So it's expanding the variable twice, which without that highlighting would be deeply mysterious, and the " quotes get very confusing when mixed in with shell quoting.

A simple example to try out :

% perl -E '$foo=bar; say qq[$foo];'
bar
%

Upvotes: 1

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