[reddit] support comment share link (#8434)

gallery_dl/extractor/reddit.py

@@ -317,8 +317,8 @@ class RedditSubmissionExtractor(RedditExtractor):
     """Extractor for URLs from a submission on reddit.com"""
     subcategory = "submission"
     pattern = (r"(?:https?://)?(?:"
-               r"(?:\w+\.)?reddit\.com/(?:(?:r|u|user)/[^/?#]+"
-               r"/comments|gallery)|redd\.it)/([a-z0-9]+)")
+               r"(?:\w+\.)?reddit\.com/(?:(?:(?:r|u|user)/[^/?#]+/)?"
+               r"comments|gallery)|redd\.it)/([a-z0-9]+)")
     example = "https://www.reddit.com/r/SUBREDDIT/comments/id/"
 
     def __init__(self, match):