[急]求正则表达式? 高手请帮忙!
字符串如下

var   str= " <P> &nbsp; </p> 请帮忙 <P> &nbsp; <P> &nbsp; </p> "

用正则表达式替换成如下结果:
<P> &nbsp; </p> 请帮忙 <P> &nbsp; </p> <P> &nbsp; </p>

也就是   " <P> &nbsp; "   替换成 " <P> &nbsp; </p> "
但是 " <P> &nbsp; </p> "   不替换.

按我的换法  

str.replace(/ <P> &nbsp;/gi, " <P> &nbsp; </p> ")
结果是:   " <P> &nbsp; </p> </p> 请帮忙 <P> &nbsp; </p> <P> &nbsp; </p> </p> "

我要的结果是   <P> &nbsp; </p> 请帮忙 <P> &nbsp; </p> <P> &nbsp; </p>


------解决方案--------------------
var str= " <P> &nbsp; </p> 请帮忙 <P> &nbsp; <P> &nbsp; </p> ";
str= str.replace(/ <P> &nbsp;( <\/P> )?/ig, " <P> &nbsp; <\/p> ");
alert(str);