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<?php preg_match('/^([a-z0-9_]+)((:?::)([a-z0-9_]+))+/', 'lol::whut::gee::loo', $matches); var_dump($matches);
based on SSAJW
Output for 4.3.0 - 7.1.0
array(5) { [0]=> string(19) "lol::whut::gee::loo" [1]=> string(3) "lol" [2]=> string(5) "::loo" [3]=> string(2) "::" [4]=> string(3) "loo" }