$re = '/\*\*Node[\s\S]*?(?=\*End Instance)/m';
$str = '** ASSEMBLY
**
*Assembly, name=Assembly
**
*Instance, name=Part-1-1, part=Part-1
*Node
**Node
1, 12., 0.
2, 12., -6.
3, 9., -15.
4, 7.99721575, -53.
** Section: t3
*Shell Section, elset=Set-3, material=PET
2., 5
** Section: t4
*Shell Section, elset=Set-4, material=PET
2., 5
*End Instance
**
*End Assembly
**
** MATERIALS
**
*Material, name=PET';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php