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<?php $data = '10 x 2,5'; echo preg_replace_callback('/(\d+(?:[.,]\d+)?)\h*x\h*(\d+(?:[.,]\d+)?)/', function($match) { return str_replace(',' , '.', $match[1]) * str_replace(',' , '.', $match[2]); }, $data);

preferences:
43.65 ms | 1751 KiB | 5 Q