[youku] Allow shortcut youku:ID and make non-matching groups non-matching (#1571)

youtube_dl/extractor/youku.py

@@ -13,7 +13,7 @@ from ..utils import (
 
 
 class YoukuIE(InfoExtractor):
-    _VALID_URL =  r'(?:http://)?(v|player)\.youku\.com/(v_show/id_|player\.php/sid/)(?P<ID>[A-Za-z0-9]+)(\.html|/v.swf)'
+    _VALID_URL =  r'(?:(?:http://)?(?:v|player)\.youku\.com/(?:v_show/id_|player\.php/sid/)|youku:)(?P<ID>[A-Za-z0-9]+)(?:\.html|/v\.swf|)'
     _TEST =   {
         u"url": u"http://v.youku.com/v_show/id_XNDgyMDQ2NTQw.html",
         u"file": u"XNDgyMDQ2NTQw_part00.flv",