<?php
$string = "Is is the cost of of gasoline going up up";
$pattern = "http://b([a-z]+) //1/b/i"; //這里的//1不能使用/$1或$1
$str = preg_replace($pattern, "http://1", $string); //這里的//1可以使用/$1或$1，引用第一個子匹配
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格式為：$0<BR>替換后的格式為：$1年$2月$3日 $4時$5分$6秒";
print preg_replace($pattern, $replacement, $time);
?>