my $utf8_decodable_regex =
    qr/[\xC0-\xDF][\x80-\xBF]    | # 2 bytes unicode char
       [\xE0-\xEF][\x80-\xBF]{2} | # 3 bytes unicode char
       [\xF0-\xFF][\x80-\xBF]{3}/x;

sub contains_decodable_utf8 { $_[0] =~ /$utf8_decodable_regex/; }

sub is_utf8_decodable {
    $_[0] =~ /\A($utf8_decodable_regex|[[:ascii:]])*\z/;
}