my %hash=(qr{([0-4])}=>"foo", a=>"foo2");
$_=2;
for my $re (keys %hash) {
P "re=%s, ref=%s", $re, ref $re;
P "match:(%s) %s", $1, $hash{$re} if /$re/;
}
'
re=a, ref=
re=(?^:([0-4])), ref=
match:(2) foo
[download]

Also, the '^' being inserted after '(?' looks odd since the original 'qr' expression didn't include it. Do you know why it was inserted in the stringified version?

At this point, to check whether or not I need to use the 'key' directly or use it in a RE it seems I look for the paren, question-mark, which just looks strange, even though it works.

... if you put [a Regexp object value produced by the qr// operator] directly into the hash and do a ref on the key you pull out, the ref-type is lost.

That's because all hash keys are (pure) strings. See what you get from the statement
    my %hash = ( [ 1, 'x', 99, ] => 'foo' );
The anonymous array reference has been stringized into something like "ARRAY(0xabc456)" and its ref-itude has been entirely lost. That's ok, because the  =~ qr// m// s/// operators et al are quite happy to operate with strings as well as on strings. I don't understand why you need to know if a given string was "originally" a Regexp object or not. What difference does it make?

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my %test = (
   qr{ f [eio]+ e }xms => 'regex 1',
   qr{pure string}     => 'regex 2',
   'pure string'       => 'string 1',
   );
 pp \%test;
 ;;
 for my $str (
   'a pure string it is, yes!',
   'a pure    string too',
   'xxx fiiiie yyy',
   ) {
   print qq{'$str'};
   ;;
   for my $k (keys %test) {
     die $k, ' not a pure string: reference ', ref $k if ref $k;
     printf qq{$test{$k}: $k: };
     print $str =~ /($k)/ ? qq{match, \$1 is '$1'} : 'NO match';
     }
   print '';
   }
"
{
  "(?^:pure string)"     => "regex 2",
  "(?^msx: f [eio]+ e )" => "regex 1",
  "pure string"          => "string 1",
}

'a pure string it is, yes!'
regex 1: (?^msx: f [eio]+ e ): NO match
string 1: pure string: match, $1 is 'pure string'
regex 2: (?^:pure string): match, $1 is 'pure string'

'a pure    string too'
regex 1: (?^msx: f [eio]+ e ): NO match
string 1: pure string: NO match
regex 2: (?^:pure string): NO match

'xxx fiiiie yyy'
regex 1: (?^msx: f [eio]+ e ): match, $1 is 'fiiiie'
string 1: pure string: NO match
regex 2: (?^:pure string): NO match
[download]

... the '^' being inserted after '(?' ... Do you know why it was inserted in the stringified version?

From Perl version 5.14 on, this represents a default set of regex modifiers that can be altered by further modifiers; see  (?^alupimsx) (update: and (?^aluimsx:pattern)) in Extended Patterns.

Update: I posted this reply before reading kcott's earlier reply. We've made a number of similar points, but one important point kcott made that I did not is "There are certain things about qr that are non-standard and non-intuitive..." It's possible to produce strings with the  q// or  qq// operators that are exactly equivalent in their effect to the stringization of the Regexp object produced by the  qr// operator, but it can be tricky. One thing is certain: the automatic stringization of a Regexp object can always be used in place of the object by the   =~  m//  s/// operators with exactly the same effect.

---

Give a man a fish:  <%-{-{-{-<

Re: the default set of regex modifiers... ahhh.. forgot about that thanks for reminding me.