$re = '/(process started).*?(process id \d{2}).*?(?<!process started)(process completed)/ms';
$str = 'process started
process id 00
some process details
process completed
process started
process id 01
some process details
process completed';
preg_match_all($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