求一正则表达式解决办法

$s = 'baid,baidu,baidu,google,baidu  baidu  sdfsdf www.baidu.com ';
echo preg_replace("/(?\\1",$s);

------解决方案--------------------
$search  = array("/(.*)([^\.]baidu[^\.])(.*)/", "/(.*)([^\.]baid[^\.])(.*)/", "/(.*)([^\.]google[^\.])(.*)/");
$replace = "\$1 baidu $3";
$str = ' baidu baid  12312 google ';
$res = preg_replace( $search, $replace, $str );
exit($res);


------解决方案--------------------
貌似PHP里有反向预查,那样的话,很简单的就可以解决了...:D
飘走...