$re = '/(?i)^(?:https?:\/\/)(?:w{3}\.)?([^\r\n]+?)\.([a-z0-9]{2,6}(?:\.[a-z0-9]{2,6})?)\/?(.*)$/m';
$str = 'https://stackoverflow.com/questions/ask
https://stackoverflow.co.uk/questions/ask
https://www.stackoverflow.com/questions/ask
http://www.stackoverflow.co.uk/questions/ask
http://www.stackoverflow.co.uk/
http://www.stackoverflow.co.uk';
$subst = "\2/\1/\3";
$result = preg_replace($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