#!/usr/bin/perl -CS

use strict;
use warnings;
use Encode qw(decode);

$_ = <DATA>;
print;

while (/(\\u(d8[0-9a-f]{2})\\u(d[c-f][0-9a-f]{2}))/ ) {  ## NB: Match only surrogate pairs
    my $rplc = $1;
    my $sp = pack( "nn", hex($2), hex($3) );
    s/\Q$rplc/decode( "UTF-16BE", $sp )/e;
    print;
}
while (/(\\u([0-9a-f]{4}))/ ) {
    my $rplc = $1;
    my $cp = pack( "n", hex($2) );
    s/\Q$rplc/decode( "UTF-16BE", $cp )/e;
    print;
}
__DATA__
Ren\u00e9 \ud83d\ude06 Fran\u00e7oise