- preg_replace: documentation ( source)
<?php
$strings = 'https://www.instagram.com/p/BarUcqwht_u';
$searchs = '~(?:https://(?:www\.)?instagram\.com/p/)?([a-zA-Z0-9_\-+?:]+)~';
$replaces = 'https://instagram.com/p/$1/media/?size=l';
$soc_instagram = preg_replace($searchs,$replaces,$strings);
echo $soc_instagram;