<?php
$str = "<img src=\"http://w...content-available-to-author-only...e.com/img/img.png\" alt=\"A Title\">";
$re = '~(<img\s*src=".*\/).*(\.[^"]*)("\s*alt="([^"]+).*)~';
echo $filtered_string;
PD9waHAKCiRzdHIgPSAiPGltZyBzcmM9XCJodHRwOi8vdy4uLmNvbnRlbnQtYXZhaWxhYmxlLXRvLWF1dGhvci1vbmx5Li4uZS5jb20vaW1nL2ltZy5wbmdcIiBhbHQ9XCJBIFRpdGxlXCI+IjsKCiRyZSA9ICd+KDxpbWdccypzcmM9Ii4qXC8pLiooXC5bXiJdKikoIlxzKmFsdD0iKFteIl0rKS4qKX4nOwoKcHJlZ19tYXRjaCgkcmUsICRzdHIsICRtKTsKCiRmaWx0ZXJlZF9zdHJpbmcgPSAkbVsxXSAuIHN0cl9yZXBsYWNlKCIgIiwiXyIsc3RydG9sb3dlcigkbVs0XSkpIC4gJG1bMl0gLiAkbVszXTsKCmVjaG8gJGZpbHRlcmVkX3N0cmluZzsK