$re = '/\/category\/(\w+\/)?(\d+)/';
$str = 'https://m.aaa.kr/category/outer/55/
https://m.aaa.kr/category/inner/5/
https://m.aaa.kr/product/jacket/3031/category/55/display/1/
https://m.aaa.kr/product/shirts/30/category/5/display/1/';
preg_match_all($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