Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Fellow Monks,

I've got the following snippet: 
#!/usr/bin/perl

my $test = qq(
    func(1);
    func(2);
    func(3);
    func(4);
);

print $test;

[download]
which prints out the following: 
    func(1);
    func(2);
    func(3);
    func(4);

[download]
My question is, why didn't the print end here:
func(1);
How did perl know how to go to the last );
??
Thanks,
Jonathan

Replies are listed 'Best First'.
Re: qq question
by inman (Curate) on Oct 18, 2005 at 15:16 UTC
    From the docs:

    Non-bracketing delimiters use the same character fore and aft, but the four sorts of brackets (round, angle, square, curly) will all nest, which means that q{foo{bar}baz} is the same as 'foo{bar}baz'

    The code falls into this category because the parens are balanced. If you unbalanced the parentheses(sp?) then you would end up with a syntax error.

[reply]
[d/l]
[select]
Re: qq question
by swampyankee (Parson) on Oct 18, 2005 at 15:29 UTC
    qq (...) quotes everthing between delimiters, but you knew that. Perl is keeping track of the parentheses nesting depth, so there is something like this going on: 
    $string = qq(           # start the quote
             func(1     # push down to second level
                   ln(  # push down to third level
                      e) # pop up to second level
                  );     # pop up to first level
            );           # finish the quote

    [download]
    After all, Perl's parser has to do something similar with an equation such as 
    
$a = $b + $c * ($d + $e * ($f + $g * ($h +)));

    [download]

    emc

[reply]
[d/l]
[select]
Re: qq question
by Moron (Curate) on Oct 18, 2005 at 16:47 UTC
    I think the underlying cause is that the '(' in the expression 'qq(' is being declared as a string delimiter but one which maintains the nesting rules of a bracket.

    Revealing tests cases include (with their effects):

    1) try with too many right brackets (perl will generate a syntax error on an attempt to nest to a depth of -1).

    2) try with too many left brackets (perl will complain using words like 'can't find a string terminator ")"'

    3) use a different nesting or non-nesting delimiter from the content and mismatch the brackets in the content as before, e.g. qq{(((((()} (perl will have no problem with this despite the mismatching brackets, because they are of a different type than the delimiter being declared for qq.)

    -M

    Free your mind

[reply]
Re: qq question
by pg (Canon) on Oct 18, 2005 at 17:14 UTC

    If you run this:

    use strict;
use warnings;

my $test = qq(a (b c d);

print $test;

    [download]

    You get this error:

    Can't find string terminator ")" anywhere before EOF at math1.pl line 
+4.

    [download]

    Which clearly indicates that perl follows the depth (matching) of '()' pairs.

[reply]
[d/l]
[select]
Re: qq question
by ioannis (Abbot) on Oct 18, 2005 at 15:25 UTC
    The qq operator is not a macro text substitution. The purpose here is (among other things, see perlop) to enable interpolation. The relevant section in perlop is 'Gory details of parsing quote-like constructs'.
[reply]

Back to Seekers of Perl Wisdom