$re = '/(?<!Account n.:\s\d{3}.)(?<!(\d|\/))\d{7}(?!.*has been received and is being reviewed by)(?!.*has been solved. To reopen)(?!\w)/m';
$str = 'Account n.: 243-479688.71J
Your request (443760) has been solved. To reopen this request, reply to this email..
Article DOI: 10.3389/fchem.2020.500600
Article DOI: 10.3389/fchem.2020.5006001
Article DOI: 10.3389/fchem.2020.50060';
preg_match_all($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