$re = '/(?<=\>)(\d{4}\-\d{2}\-\d{2}T\d{2}\:\d{2}\:\d{2}Z)(?=<\/span)/';
$str = '<p><span data-date="2019-05-10" data-time="19:00:00" class="discourse-local-date" data-timezones="Europe/Brussels" data-timezone="Europe/Berlin" data-email-preview="2019-05-10T17:00:00Z UTC">2019-05-10T17:00:00Z</span> → <span data-date="2019-05-10" data-time="22:00:00" class="discourse-local-date" data-timezones="Europe/Brussels" data-timezone="Europe/Berlin" data-email-preview="2019-05-10T20:00:00Z UTC">2019-05-10T20:00:00Z</span><br>
<div class="lightbox-wrapper"><div class="meta">
<span class="filename">HackSpace_by_Sugar_Ray_Banister.jpg</span><span class="informations">1596×771 993 KB</span><span class="expand"></span>
</div></a></div></p>';
preg_match_all($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