$re = '/(?s)\biPhone:\n\n+(?:(?!\n\n).)+\bSerial Number: (\w+)/m';
$str = 'USB:
USB 2.0 SuperSpeed Bus:
Host Controller Location: Built-in USB
Internal Memory Card Reader:
Product ID: 0x8406
Serial Number: 000000000820 //i dont want this
Built-In: Yes
USB 3.0 Hi-Speed Bus:
PCI Device ID: 0x8c31
iPhone:
Vendor ID: 0x05ac (Apple Inc.)
Version: 7.02
Serial Number: wea0aa752ada7722ac92575e98z2e89c691f4282 //i want this
Speed: Up to 480 Mb/sec
Manufacturer: Apple Inc.
Location ID: 0x14100000 / 9
Apple Internal Keyboard / Trackpad:
Product ID: 0x0262
Vendor ID: 0x05ac (Apple Inc.)
Location ID: 0x14c00000 / 3
Current Available (mA): 500
Current Required (mA): 40
Built-In: Yes';
preg_match_all($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