<?php
$string ='<img src="image.jpg" alt=212><img src="http://u...content-available-to-author-only...a.org/wikipedia/commons/e/e5/Raster_to_Vector_Mechanical_Example.jpg" alt=45>';
$pattern = '/(<img src=\")(?!http)/i';
echo $content;
PD9waHAKCiRzdHJpbmcgPSc8aW1nIHNyYz0iaW1hZ2UuanBnIiBhbHQ9MjEyPjxpbWcgc3JjPSJodHRwOi8vdS4uLmNvbnRlbnQtYXZhaWxhYmxlLXRvLWF1dGhvci1vbmx5Li4uYS5vcmcvd2lraXBlZGlhL2NvbW1vbnMvZS9lNS9SYXN0ZXJfdG9fVmVjdG9yX01lY2hhbmljYWxfRXhhbXBsZS5qcGciIGFsdD00NT4nOwokcGF0dGVybiA9ICcvKDxpbWcgc3JjPVwiKSg/IWh0dHApL2knOwokY29udGVudCA9IHByZWdfcmVwbGFjZSgkcGF0dGVybiwgJyQxTkVXIScsICRzdHJpbmcpOwoKZWNobyAkY29udGVudDsK