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

Why is $$r not defined? I'm not sure what value I expected to find at the end of ${qr//} but I didn't think it'd be undef.

use strict;
use warnings;

my $r = qr/Delectable/;
print defined $$r ? "Defined" : "Not defined";

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Re: Undefined regexp objects?
by ihb (Deacon) on Jan 16, 2003 at 00:12 UTC
    For those that wonder how the heck you even can dereference a qr// object: I'm sorry, I don't know either. :)

    But this below is usually how I go about when I find an unexpected behaviour of this kind. I perhaps won't get all the whys, but I might get an explanation that works for me and that will add another piece to the puzzle.

    I do believe that perl interally treats a qr// as a blessed scalar, being blessed into class Regexp. The object itself is as magical as objects get I think, and we can't get any useful info out of just printing the object as you usually can, but overload can help us out: 
      use overload;
  print overload::StrVal(qr/asdf/);
  # Regexp=SCALAR(...)

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    And Devel::Peek help us in another way: 
      use Devel::Peek;
  Dump(qr/asdf/);

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    yielding: 
    SV = RV(0x1af1fd8) at 0x1b0f118
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,ROK)
  RV = 0x1b0f190
  SV = PVMG(0x1c2c5ac) at 0x1b0f190
    REFCNT = 1
    FLAGS = (OBJECT,RMG)
    IV = 0
    NV = 0
    PV = 0
    MAGIC = 0x1af012c
      MG_VIRTUAL = 0x2808c138
      MG_TYPE = 'r'
      MG_OBJ = 0x1b0b51c
    STASH = 0x1de7570   "Regexp"

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    showing us that regexes are magical. I don't see it being defined anywhere. But I have no idea why this is the case. But I can't say I'd expect a sane dereference value for a magical object. :) In fact, thinking about it I'd more expect undef than anything else because... the value isn't defined since it's not actually a scalar, and what would be a good substitute value anyway?

    Hoped this helped in some way,
    ihb
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Re: Undefined regexp objects?
by Paladin (Vicar) on Jan 15, 2003 at 23:39 UTC
    qr// doesn't return a reference. Try: 
    use strict;
use warnings;

my $r = qr/Delectable/;
print $r;

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    Update: ihb is quite right. I should have qualified that as scalar reference.
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      qr// does indeed return a reference, try 
        print ref(qr//)

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      This object stringifies through, so you can interpolate it into other patterns.

      ihb
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        In fact... you can also call methods off of it and rebless the object into a different package and it keeps it's Regexp nature.

        use strict;
use warnings;

my $r = qr/test/;
print $r -> match( 'test' ) ? "Match\n" : "No match\n";
bless $r, 'beans';
print $r -> match( 'test' ) ? "Match\n" : "No match\n";

package beans;
sub match { $_[1] =~ $_[0] }

package Regexp;
sub match { $_[1] =~ $_[0] }

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Re: Undefined regexp objects?
by pg (Canon) on Jan 16, 2003 at 05:35 UTC
    The comments explains what this piece of code is doing: 
    use B;

$foo = qr/foo/;
if ($foo->isa("Regexp")) {#check whether it is a Regexp object
    print "is a Regexp\n";
} else {
    print "is not a Regexp\n";
}

print B::svref_2object($foo)->MAGIC->TYPE;#if it is not a sv ref, perl
+ will not allow you to call svref_2object

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