When $u is null, null is passed to preg_match() which is deprecated behavior since PHP 8.1.
Thus first check if $u === null.

ERROR 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [/var/www/html/phorge/arcanist/src/parser/xhpast/api/XHPASTNode.php:243]

Refs T15926

Either read the code, or probably run something like arc unit on D25797?