what it says. this is valid syntax since 5.2.2.
Search for ?P< in core, where you find it replace it with ?' and the matching > with '. I think there are four. Leave Symfony alone. That's all.
It is possible to name a subpattern using the syntax (?Ppattern). This subpattern will then be indexed in the matches array by its normal numeric position and also by name. PHP 5.2.2 introduced two alternative syntaxes (?pattern) and (?'name'pattern).