regex preg_match for digit in middle for a string

Question

ddw_0_op1
ddw_1_op1
ddw_2_op1
ddw_3_op1
ddw_4_op1

in these strings of array how i can extract or preg_match only the number in middle that increase . t

Answer 1

I would break the string on underscore:

$parts = explode('_', $string);
$middle = $parts[1];