<?php
$re = '/^http(?:s)?:\/\/\K([^\/]+\/[^\/]+\/).*$/m';
$str = 'http://e...content-available-to-author-only...e.com/a/b/c.php';
echo $matches[1];
?>
PD9waHAKCiRyZSA9ICcvXmh0dHAoPzpzKT86XC9cL1xLKFteXC9dK1wvW15cL10rXC8pLiokL20nOwokc3RyID0gJ2h0dHA6Ly9lLi4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5lLmNvbS9hL2IvYy5waHAnOwpwcmVnX21hdGNoKCRyZSwgJHN0ciwgJG1hdGNoZXMpOwplY2hvICRtYXRjaGVzWzFdOwoKPz4=