我想知道哪里不对了，关于 preg_replace

[jackerbauer]

$a = '?ac=list&typeid=354&op=down&page=8';
print preg_replace("/?ac=list(&)?typeid\=(\d+)?(&)?op\=(\w+)?(&)?page\=(\d+)?/i","\$2",$a);

Warning: preg_replace() [function.preg-replace]: Compilation failed: nothing to repeat at offset 0 in D:\APMServ5.2.6\www\htdocs\king.php on line 40

可是我就是找不到错误了！！

[xuzuning]

$a = '?ac=list&typeid=354&op=down&page=8';
print preg_replace("/\?ac=list(&)?typeid\=(\d+)?(&)?op\=(\w+)?(&)?page\=(\d+)?/i","\$2",$a);

[igaojie]

转义～！··