[SPOILER] Expert #12

#
# recursive pattern/function
#

our ($sequence, $pattern);
{
    my $fail  = qr/(?{ die "Malformed escape sequence: '$_'\n" })/;

    my $esc   = qr/ \\(?=[\\<>]) /x;
    my $code  = qr/ [A-Za-z0-9] /x;
    my $body  = qr/ (??{ $sequence }) | \\. | [^\\>] /x;

    $sequence = qr/ ($code) < ($body*) (?: > | \Z $fail) /x;
    $pattern  = qr/ $sequence | ($esc) /x;
}

sub expand_escapes {
    my ($string, $map) = @_;

    my $replace = sub {
        my ($code, $body, $esc) = ($1, $2, $3);

        return "" if $esc;

        my $inside = expand_escapes($body, $map);
        if (my $func = $map->{$code}) {
            return $func->($inside);
        }

        warn "Undefined escape code: '$code'\n";
        return $inside;
    };

    $string =~ s{ $pattern }{ &$replace }exg;
    $string
}