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run code in 200+ php & hhvm versions
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<?php echo preg_replace('/#[a-z]\w+/i', '<a href="#">$0</a>', '#11 #f1 #1f #ff');
Output for 4.3.0 - 7.2.0
#11 <a href="#">#f1</a> #1f <a href="#">#ff</a>