In Object Oriented Perl, Chapter 5, TheDamian blesses a regular expression made with qr (see perlop). I never gave much thought to this, but I recently noticed the following:

What this seems to mean is that once I bless a regular expression into some other class, I can't tell that it's really a regular expression under the hood anymore, but it is. I also can't tell if some scalar reference is really a regexp or just some scalar that some joker decided to call a 'Regexp'.

The code below demonstrates what I'm talking about. In it, a real regular expression is examined and used both before and after being blessed. Next, a blessed scalar ref is examined the same way. It looks the same, but it doesn't work the same. 

use Test::More 'tests' => 25;
use Scalar::Util qw( reftype blessed );

ok( 'foo' =~ //, '"foo" matches empty expression' );

# a real regexp, not blessed, works like this
my $rx = qr/f(oo)/;
is( ref $rx, 'Regexp', 'ref $rx eq "Regexp"' );
is( blessed $rx, 'Regexp', '$rx is blessed as "Regexp"' );
is( reftype $rx, 'SCALAR', 'reftype $rx eq "SCALAR"' );
ok( ! defined $$rx, '$$rx is not defined' );
ok( 'foo' =~ $rx, '$rx matches "foo"' );
is( $&, 'foo', 'matched text is "foo"' );
is( $1, 'oo', 'first capture is "oo"' );

# a blessed regexp works the same way,
# but what other evidence can tell me it's a regexp?
bless $rx, 'NotRegexp';
is( ref $rx, 'NotRegexp', 'ref $rx eq "NotRegexp"' );
is( blessed $rx, 'NotRegexp', '$rx is blessed as "NotRegexp"' );
is( reftype $rx, 'SCALAR', 'reftype $rx eq "SCALAR"' );
ok( ! defined $$rx, '$$rx is not defined' );
ok( 'foo' =~ $rx, '$rx matches "foo"' );
is( $&, 'foo', 'matched text is "foo"' );
is( $1, 'oo', 'first capture is "oo"' );

# this looks just like the blessed regexp, but it doesn't function
my $o;
my $notrx = \$o;
bless $notrx, 'NotRegexp';
is( ref $notrx, 'NotRegexp', 'ref $notrx eq "NotRegexp"' );
is( blessed $notrx, 'NotRegexp', '$notrx is blessed as "NotRegexp"' );
is( reftype $notrx, 'SCALAR', 'reftype $notrx eq "SCALAR"' );
ok( ! defined $$notrx, '$$notrx is not defined' );
ok( !('foo' =~ $notrx), '$notrx does not match "foo"' );

# this looks just like the real regexp, but it doesn't function
bless $notrx, 'Regexp';
is( ref $notrx, 'Regexp', 'ref $notrx eq "Regexp"' );
is( blessed $notrx, 'Regexp', '$notrx is blessed as "Regexp"' );
is( reftype $notrx, 'SCALAR', 'reftype $notrx eq "SCALAR"' );
ok( ! defined $$notrx, '$$notrx is not defined' );
ok( !('foo' =~ $notrx), '$notrx does not match "foo"' );

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My question is, how can I tell if some scalar reference I receive really is a regular expression? Obviously, if it produces a match when used, I know it is a regular expression (a scalar reference to a string won't act as a regexp), but a non-match doesn't tell me much.

In reply to How can I tell if an object is based on a regex? by kyle

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