<?php

$domains = array("example.com", "main.com");
$s = "http://e...content-available-to-author-only...e.com http://m...content-available-to-author-only...n.com http://l...content-available-to-author-only...t.com";
$re = '/http(?=:\/\/(?:[^\/]+\.)?(?:' . implode("|", array_map(function ($x) {return preg_quote($x); }, $domains)) . ')\b)/i';
echo preg_replace($re, "https", $s);