fix: regex correctly matches url() referenced assets

Additionally, adjusted how greedy the regex is and added
links out to Regex101.com for easy future reference

This fix addresses the issue reported here:
getgrav#7

cdn.php

@@ -76,7 +76,11 @@ function ($matches) use ($blocks, $pullzone) {
 
         // replacements for inline CSS url() style references
         if ($config['inline_css_replace']) {
-            $regex = "/(url\()(?:" . $base . ")(.*?\.(?:" . $extensions . "))(.*;)/i";
+
+            // https://regex101.com/r/8zAnec/2 -> (url\([\'\"])(?:)(.*?\.(?:jpe?g|png|gif|ttf|otf|svg|woff|xml|js|css))(.*?\);)/i
+            // or with $base
+            // https://regex101.com/r/g0R6sj/2 -> (url\([\'\"])(?:http:\/\/github\.com)(.*?\.(?:jpe?g|png|gif|ttf|otf|svg|woff|xml|js|css))(.*?\);)/i
+            $regex = "/(url\([\'])(?:" . $base . ")(.*?\.(?:" . $extensions . "))(.*?\);)/i";
 
             $this->grav->output = preg_replace_callback(
                 $regex,