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<?php var_dump(preg_replace('/(?<=[a-z])([A-Z])/L', '_$1', 'AnaNaaZ'));
based on 8WqW4
Output for 4.3.11 - 7.1.0
Warning: preg_replace(): Unknown modifier 'L' in /in/5CknE on line 3 NULL
Output for 4.3.0 - 4.3.10
Warning: Unknown modifier 'L' in /in/5CknE on line 3 NULL