困惑（正则表达式）

<?php
$mod = '/\\/';
$str ="\\a\b\\cd\"";
echo $str;
preg_match_all($mod,$str,$arry);
print_r($arry);
?>
输出结果：*******************************************
\a\b\cd"
Warning: preg_match_all() [function.preg-match-all]: No ending delimiter '/' found in Regular.php on line 5
****************************************************
<?php
$mod = '/\\\\/';        ***'/\\\/'***结果一样
$str ="\\a\b\\cd\"";
echo $str;
preg_match_all($mod,$str,$arry);
print_r($arry);
?>
输出结果：************************************
\a\b\cd"Array ( [0] => Array ( [0] => \ [1] => \ [2] => \ ) )
*******************************************************

<?php
$mod = '/\\\\\\\\/';
$str ="\\a\\b\\\cd\"";
echo $str;
preg_match_all($mod,$str,$arry);
print_r($arry);
?>
输出结果****************************
\a\b\\cd"Array ( [0] => Array ( [0] => \\ ) )
****************************************

why??? why????why??????????????

这需要两层转义的，先是字符串转义，然后正则的匹配模式转义

谢谢，我找了些资料，明白了