$url = "http://www.nowamagic.net/index.html";
preg_match("/^(http:\/\/)?([^\/]+)/i", $url, $matches); 
print_r($matches['0']);

$str = "<p><img src='/data/upload/meinv/2016/09/meinv_2016090216224620142ae2baeee8f44d36b3eea01a870c.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_2016090221322097e8532da44506df8d3a3c8a03f9663e.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_201609022135394ee7d4eb9d457a0555a5b84a23c3586e.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_20160902213522a9e5bcd90fad7e02dcea58a17d5af39b.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_20160902213514e657e6f665a13034d31cd187c8ce1101.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_201609022135094a52bc60996292565257a5a0c97ffc2d.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_201609022132373677c34ce9d8a180da33e44092cf70da.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_20160902213228dfb3037aa666e44456f353138c7cd2a9.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_201609022132220a0c649bca9757e767ccb18cfee749da.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>_ueditor_page_break_tag_<p><img src='/data/upload/meinv/2016/09/meinv_20160902213221d99342b311cbf2b79ce6e94f805e02e3.gif' alt='斋藤飞鸟gif动态图合集 好甜美的混血美女斋藤飞鸟'></p>";

$pattern="/<[img|IMG].*?src=[\'|\"](.*?(?:[\.gif|\.jpg]))[\'|\"].*?[\/]?>/"; 
preg_match_all($pattern,$str,$match); 
print_r($match);