两个下划线之间的数字的正则表达式

I am trying to find the number between two underscores (_) in this string

234534_45_92374
3433_9458_034857
zx_8458_047346daf

what would be the regex for this?

preg_match('/_(\d+)_/', $str, $matches);
$number = (int) $matches[1];