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run code in 150+ php & hhvm versions
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<?php /*echo preg_replace_callback('/([a-z]+)(_[a-z]+)/', function($u) { var_dump($u); return ucwords(str_replace('_', '', $u[1])); } , 'rob_tr_yuui'); */ echo preg_replace('/([a-z]+_[a-z]+)/', '$1.$2', 'rob_tr_yuui');
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
rob_tr._yuui