I don't understand a piece of this code

[Socrates440]

I am working my way through a beginning perl text book in my free time and I am currently reading the chapter on regular expressions. Anyways, I am confused by a piece of this code:

$what = "[box]";
foreach (qw(in[box] out[box] white[sox])) {
if (/\Q$what\E/) {
print "$_ matched!\n";
}
}

I get that it is storing the string \box\ in the varriable $what. If I understand it right, the foreach statement parses each of the strings in the parenthesies looking for the string \box\. What is the "qw" in the line "(qw(in\box\ out\box\ white\sox\))" though? I have no idea what that is. Sorry about the words box and sox being highlighted in red. I don't know how to stop this site from interpolating (correct word?) the brackets.

[bitingduck]

The "qw" just stands for "quote words" for quoting a list of words. Which is pretty much what you inferred.

I don't see anything in red though-- code tags generally do a good job keeping the site from doing strange things to code

[Albannach]

I rather think of qw as quote whitespace since what it does is break up the subject string using whitespace as separators. So the string inside the qw() is broken into 3 pieces, not 3 words as in this case each resulting piece contains 2 words and some square brackets. See perlop under Quote and Quote-like operators.

The next think you need to know is that \Q and \E are allowing you to use the content of the variable $what as literal text without interpreting any special characters, which in this case are the square brackets. Have a look at perlre to see what [box] means inside a regular expression.

So in summary it is looking for the string [box] including the square brackets, withing the 3 separate strings in[box], out[box], and white[sox], and prints matches.

[bitingduck]

That's how I initially learned it as well, but perldoc.perl.org j ust says "quote a list of words" where you have to sort of liberally accept that a word is letters separated by whitespace. "Quote with whitespace" makes more sense to me than "quote whitespace", but it mostly just matters that it works as advertised.

[cheekuperl]

The code is doing what you've understood:

$what = "[box]";
foreach (qw(in[box] out[box] white[sox])) { #Assign to $_, one by one,
+ each value in parenthesis
if (/\Q$what\E/) { 
print "$_ matched!\n";

\Q and \E disable special meaning of all metachars in between them. See perlre

[Anonymous Monk]

Remember I am geting an unhelpful error message. Not sure how to debug it. ?

Remember Tutorials: Basic debugging checklist , brian's Guide to Solving Any Perl Problem?

Second verse, same as the first, go fishing :)

To see if your code is what you thought it was, use B::Deparse

$ perl -MO=Deparse,-p crates
($what = '[box]');
foreach $_ (('in[box]', 'out[box]', 'white[sox]')) {
    if (/\Q$what\E/) {
        print("$_ matched!\n");
    }
}
crates syntax OK

Hopefully that code you should recognize ( perlintro if you don't)

For a word explanation see perlop#Quote and Quote like Operators, qw/STRING/, Quote and Quote-like Operators, Modern Perl pdf page 35 :)

I don't know how to stop this site from interpolating (correct word?) the brackets.

use code ( <c>[box]</c> ) tags or html enities, see PerlMonks FAQ, Markup in the Monastery

See also How do I compose an effective node title?, an effective title might "What is qw()?" or "How does qw() work?"

[jamesbutler]

You may find this website helpful http://perlmeme.org/howtos/perlfunc/qw_function.html