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<?php var_dump(preg_replace('/(?<=[a-z])([A-Z])/', '_$1', 'AnaNaaZ'));
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
string(9) "Ana_Naa_Z"