求php一条preg_match_all正则，取指定div的id开头？

div的id规则如下
我要取id的以num_开头_off为结尾的div

<div id="num_sfew151245_off">
<div id="num_rewr5464511_off">
<div id="num_dvivfd15_off">

我这样写会报错误

$reg = "/<div id=\"num_.*?_off\".*?>.*?<\/div>/ism";

$reg = "/<div id=\"num_(.*?)_off\".*?>.*?<\/div>/ism";