$re = '~(?<=/)\d+~sx';
$str = 'https://www.flickr.com/photos/sdasmarchives/8004393890/in/photolist-dcjBrd-qJX244-cnT1Um-bUBvH1-avgira-dcjERJ-8bjWSe-r4UAfF-7h1iuk-SXsidX-bF9Auf-pvnyFe-dcjADK-dV8DPd-dKcBhS-qvJWV3-cnkfdS-pMX4Ym-dcjBji-dV3a4L-dcjByA-dcjykA-8bohud-cnT1ry-cS8CpN-7v1YJ3-qeov7k-bbpFBX-7ZAkCX-dF1y7D-hHPnWc-e8QdTV-6fMZTv-7ZAkDF-6K4JrR-hkHtCY-aWjBu6-5QuCU2-dcjETE-8izDu5-dhZFgR-rmk5oB-TjSLSy-6HAhJL-f7X3AF-TZGwG1-bUwueL-evVBry-dgDH5e-dcjxsT/';
preg_match_all($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