[pm-h] Using Moose Attributes In Regular Expressions - Question Regarding Magic ${\( ... )}

[Julian Brown]

I am not familiar with Moose, but am not convinced this is specific to
Moose.

I assume $object->foo is really a method call that returns the foo
attribute?  Or is it like a hash value?


• [root at julian64:~/work]# cat doit.pl
#!/usr/local/cpanel/3rdparty/bin/perl

use strict;
use warnings;

my $var = "A";
my $var_ref = \$var;

print "VAR :${var}:\n";
print "VAR VAR :${$var_ref}:\n";

Output is:

VAR :A:
VAR VAR :A:

I think we are in the same realm perhaps the parens inside the ${} is
necessary to execute the method?

Julian



On Tue, Aug 23, 2016 at 2:02 PM, Robert Stone via Houston <houston at pm.org>
wrote:

> Greetings,
>
> I find myself needing to use the value of a Moose Attribute in a regular
> expression every now and then.  Typically I accomplish this via (warning
> that all examples are very contrived and may contain bugs):
>
> *Sample Using Variable Assignment First*
>
> my $value = $my_moose_object->attribute;
> if( $some_value =~ m/\Q$value/ ) {
>
> Needless to say this isn't the most efficient/easiest to work with.  Given
> that fact, I've come across a way of using Moose Attributes directly in
> regular expressions:
>
> *MyObject*
>
> package MyObject;
>
> use Moose;
>
> has foo => ( is => 'ro', isa => 'Str' );
>
> ...;
>
> *Script That Consumes MyObject*
>
> my $object = MyObject->new( foo => 'Value' );
> if( $some_value =~ m/\Q${\( $object->foo )}\E/ ) {
>
>
> This works, but frankly I'm not entirely certain why.  From the
> documentation at http://perldoc.perl.org/perlre.html#Regular-Expressions
> I have:
>
> \Q          quote (disable) pattern metacharacters until \E
> \E          end either case modification or quoted section, think vi
>
> Great, that makes sense.
>
> But what magic is the ${\( ... )} doing here?  I'd be most grateful if
> anyone had some insight and could share it with us!
>
> Best Regards,
> Robert Stone
>
>
>
> _______________________________________________
> Houston mailing list
> Houston at pm.org
> http://mail.pm.org/mailman/listinfo/houston
> Website: http://houston.pm.org/
>
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