$re = '/href="https:\/\/my\.master\.com\/@([^\/]*)\/([^\/]*)\/([^\/]*)\/"/m';
$str = 'href="https://my.master.com/@top-com/d/my-zen/"
href="https://my.master.com/@top-com/d/my-zen/my-zen/"
href="https://my.master.com/@top-com/d/my-zen/my-zen/my-zen/"';
$subst = "href=\"https:__my.master.com_@$1_$2_$3_.html\"";
$result = preg_replace($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