为什么这个PHP preg_match返回一个空数组？

Question

$log = '127.0.0.1 - - [27/Feb/2014:18:29:38 -0800] "OPTIONS * HTTP/1.0" 200 -';
$regex = '/^\[([^\]]+)\] \[([^\]]+)\] (?:\[client ([^\]]+)\])?\s*(.*)$/i';
preg_match($regex, $log, $matches);

print_r($matches);  

有谁知道为什么这个preg_match函数只返回一个空数组....

它应该返回一个类似于解析行的数组：

Array
(
[0] => 10.1.1.150 - - [29/September/2011:14:21:49 -0400] "GET /info/ HTTP/1.1" 200 9955 "http://www.domain.com/download/" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_8; de-at) AppleWebKit/533.21.1 (KHTML, like Gecko) Version/5.0.5 Safari/533.21.1"
[1] => 10.1.1.150
[2] => -
[3] => -
[4] => 29/September/2011
[5] => 14:21:49
[6] => -0400
[7] => GET
[8] => /info/
[9] => HTTP/1.1
[10] => 200
[11] => 9955
[12] => "http://www.domain.com/download/"
[13] => "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_8; de-at) AppleWebKit/533.21.1 (KHTML, like Gecko) Version/5.0.5 Safari/533.21.1"
)