PHP正则表达式的逆向引用与子模式分析

?php
$string = "Is is the cost of of gasoline going up up";
$pattern = "/\b([a-z]+) \\1\b/i"; //这里的\\1不能使用\$1或$1
$str = preg_replace($pattern, "\\1", $string); //这里的\\1可以使用\$1或$1，引用第一个子匹配
echo $str; //效果是Is the cost of gasoline going up
?>

?php
$time=date("Y-m-d H:i:s");
$pattern = "/(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})/i";
$replacement = "\$time格式为：$0BR>替换后的格式为：$1年$2月$3日 $4时$5分$6秒";
print preg_replace($pattern, $replacement, $time);
?>