[odnoklassniki] Improve _VALID_URL readability

youtube_dl/extractor/odnoklassniki.py

@@ -19,7 +19,18 @@ from ..utils import (
 
 
 class OdnoklassnikiIE(InfoExtractor):
-    _VALID_URL = r'https?://(?:(?:www|m|mobile)\.)?(?:odnoklassniki|ok)\.ru/(?:video(?:embed)?/|web-api/video/moviePlayer/|live/|dk\?.*?st\.mvId=)(?P<id>[\d-]+)'
+    _VALID_URL = r'''(?x)
+                https?://
+                    (?:(?:www|m|mobile)\.)?
+                    (?:odnoklassniki|ok)\.ru/
+                    (?:
+                        video(?:embed)?/|
+                        web-api/video/moviePlayer/|
+                        live/|
+                        dk\?.*?st\.mvId=
+                    )
+                    (?P<id>[\d-]+)
+                '''
     _TESTS = [{
         # metadata in JSON
         'url': 'http://ok.ru/video/20079905452',