<?php
preg_match_all( "/<img.*https?:\/\/[^\/]+\/([^\"']+)[\"'].*/i", "<img src='http://google.com/test/test/test/test.png' width='test'>", $match);
echo $match[1][0];
PD9waHAKcHJlZ19tYXRjaF9hbGwoICIvPGltZy4qaHR0cHM/OlwvXC9bXlwvXStcLyhbXlwiJ10rKVtcIiddLiovaSIsICI8aW1nIHNyYz0naHR0cDovL2dvb2dsZS5jb20vdGVzdC90ZXN0L3Rlc3QvdGVzdC5wbmcnIHdpZHRoPSd0ZXN0Jz4iLCAkbWF0Y2gpOwoKZWNobyAkbWF0Y2hbMV1bMF07