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<?php echo preg_replace('/#(\b[a-zA-Z0-9]+\b)/iu', '<a href="#">#$1</a>', '#11 #f1 #1f #ff');
Output for 4.3.0 - 7.2.0
<a href="#">#11</a> <a href="#">#f1</a> <a href="#">#1f</a> <a href="#">#ff</a>