regex.php

<?php

$string1 = '<link href="/css/bs4-dynamic-fields-form-2.min.css" rel="stylesheet" media="screen">';
$string2 = '<script src="/cdn-cgi/apps/head/qx0HK83qWJ5I3r2xnETN-ETfo6s.js"></script>"'; 

$regex1 = '~<link(.*?)href="([^"]+)"(.*?)>~'; 
$regex2 = '/src=(["\'])(.*?)\1/';

$result1 = preg_match($regex1, $string1, $matches1);
$result2 = preg_match($regex2, $string2, $matches2);

var_dump($matches1[2]); 
var_dump($matches2[2]);