字符串嵌套 匹配的问题

Question

$nr="asdasdasd”、
$nr=preg_replace("/\[color=(.*)\](.*)\[\/color\]/s","<span style=\"color:\\1;\">\\2</span>",$nr);
$nr=preg_replace("/\[size=(.*)\](.*)\[\/size\]/s","<span style=\"font-size:\\1 ;\">\\2</span>",$nr);最后替换成了 
<span style="color:#FFFFFF][size=22px]asdasdasd[/size;"></span> .....我预想的结果是 
<span style="color:#FFFFFF><span style="font-size:22px"> asdasdasd</span> </span>