regex for number between two underscores

I开发者_开发百科 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?

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