use warnings;
use strict;
use open qw/:std :encoding(UTF-8)/;
use Test::More tests=>1;
use Data::Dump qw/pp/;
use Encode qw/decode/;

my $str = "Ren\\u00e9 \\ud83D\\uDe06\\uDb40\\udDeF Fran\\u00E7oise"
	." \\\\u2660\\U1234";

$str =~
s{  (?| (?&U)(d[89ab][0-9a-f]{2}) (?&U)(d[c-f][0-9a-f]{2})
      | (?&U)([0-9a-f]{4}) )
    (?(DEFINE) (?<U> (?-i) (?<!\\)(?:\\\\)*\\u ) )
}{ decode("UTF-16BE", pack("n*", hex $1, defined $2 ? hex $2 : ()),
	Encode::FB_CROAK) }iexg;

is $str, "Ren\xE9 \x{1F606}\x{E01EF} Fran\xE7oise \\\\u2660\\U1234",
	$str or diag pp $str;