What is the name of this regex feature, and is there any documenation about it (PHP specific or not)?
preg_match('/(*MARK:A)a/', 'a', $matches);
https://github.com/nikic/FastRoute/blob/master/test/Dispatcher/MarkBasedTest.php#L7
This is documented in the PCRE pattern documentation, search for "Recording which path was taken" on that page. It appears to be a fairly specific feature and it wouldn't be useful in the example you have provided.