OK, let's forget my original problem because it is already solved (Eily pointed out that before I convert a string into Regexp object, I can just print it first, and in my case it's true, all I have is a string). But sometimes I do have a Regexp object, for example in my other use-case the validation specification already contains a Regexp object, for example:

$foo_spec = {
    args => {
        arg1 => {required=>1, match=>qr/\A\w+\z/},
        arg2 => {required=>1},
    },
};
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defined($arg1) or die "arg1 must be defined";
$arg1 =~ qr/\A\w+\z/ or die "arg1 does not match regex";
defined($arg2) or die "arg2 must be defined";

The spec can be written by others.

Of course, I can require the 'match' key to have a value of string and not Regexp object, so this avoids the problem.

But I'm interested in knowing whether a newer Perl (5.14+) can stringify a Regexp object in a way that is compatible with older Perls.

Actually, my original problem with DateTime::Format::Alami is not a problem at all, because in my module I never convert to Regexp object. Sorry about that!

I have reiterated the problem in the first post.

I still don't understand your use-case well enough to see why the  "(?adlupimsx-imsx)" embedded pattern-match modifier would not be useful here (see Extended Patterns in perlre).

---

Give a man a fish:  <%-(-(-(-<

OK, let me try again. If I do this on perl 5.14+:

use 5.010;
open my($fh), ">", "out";
my $re = qr/a/;
say $fh "use 5.010; if ('a' =~ /$re/) { say 'match' } else { say 'no match' }";

Then out will contain:

use 5.010; if ('a' =~ /(?^:a)/) { say 'match' } else { say 'no match' }

This code won't run on perl 5.12 or lower due to unknown regex sequence (?^...), which is introduced in perl 5.14. When run on perl 5.12 or perl 5.10, it will print this error message:

Sequence (?^...) not recognized in regex; marked by <-- HERE in m/(?^ <-- HERE :a)/ at out line 1.

I want a code generator that, even though run from perl 5.14+, can generate code that runs on perl 5.12 and lower. So basically I now need to do something like this:

use 5.010;
use re qw(regexp_pattern);
open my($fh), ">", "out";
my $re = qr/a/;
my ($pat, $mod) = regexp_pattern($re);
say $fh "use 5.010; if ('a' =~ /(?$mod-)$pat/) { say 'match' } else { say 'no match' }";

Note that I left out the part where I need to: 1) remove or complain about regex modifiers that are new in 5.14+ and unrecognized in older perls; 2) escape slashes in pattern. But the main point is there.

UPDATE: I'm packaging the routine as a CPAN module: Regexp::Stringify