$re = '/\/[^\/]*\/[^\/]*\/$/m';
$str = 'https://www.reddit.com/r/RationalPsychonaut/comments/5m4rhv/a_satirical_blog_about_enlightenment_in_everyday/
https://www.reddit.com/r/swift/comments/5mriaj/i_just_released_a_swift_wrapper_for_the_llvm_api/
';
$subst = "/";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php