$re = '/^(.*\/thumb\/[0-9a-f]\/[0-9a-f]{2}\/)([^\/]+\.(jpg|jpeg|png|gif))\/\d+px-(\2)/im';
$str = 'https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Royal_tyrrell_entrance.JPG/800px-Royal_tyrrell_entrance.JPG
https://upload.wikimedia.org/wikipedia/commons/6/6d/Royal_tyrrell_entrance.JPG
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Royal_tyrrell_entrance.jpg/800px-Royal_tyrrell_entrance.jpg
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Royal_tyrrell_entrance.jpeg/800px-Royal_tyrrell_entrance.jpeg
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Royal_tyrrell_entrance.png/800px-Royal_tyrrell_entrance.png
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Royal_tyrrell_entrance.gif/800px-Royal_tyrrell_entrance.gif
//upload.wikimedia.org/wikipedia/commons/thumb/c/cf/Flag_of_Canada.svg/30px-Flag_of_Canada.svg.png 1.5x
<img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Royal_Tyrrell_Museum.JPG/220px-Royal_Tyrrell_Museum.JPG" decoding="async" width="220" height="165" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Royal_Tyrrell_Museum.JPG/330px-Royal_Tyrrell_Museum.JPG 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Royal_Tyrrell_Museum.JPG/440px-Royal_Tyrrell_Museum.JPG 2x" data-file-width="4000" data-file-height="3000">
<img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/9/91/Joseph_Burr_Tyrrell_bust.jpg/220px-Joseph_Burr_Tyrrell_bust.jpg" decoding="async" width="220" height="293" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/91/Joseph_Burr_Tyrrell_bust.jpg/330px-Joseph_Burr_Tyrrell_bust.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/91/Joseph_Burr_Tyrrell_bust.jpg/440px-Joseph_Burr_Tyrrell_bust.jpg 2x" data-file-width="1536" data-file-height="2048">
';
preg_match_all($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