$_ = "abc" x 4; $re_short = qr/([0-9a-f]{4}|[0-9a-f]{8})/; $re_long = qr/([0-9a-f]{8}|[0-9a-f]{4})/; print $1, $/ if /$re_short/; print $1, $/ if /$re_long/; __END__ abca abcabcab