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<?php $string = "Porcupines\nare\n\n\n\nporcupiney."; echo preg_replace('/(?:(?:\r\n|\r|\n)\s*){2}/s', "<br /><br />", $string);
Output for 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.26, 8.1.0 - 8.1.13
Porcupines are<br /><br />porcupiney.

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