[patreon] fix regex pattern for posts

The previous one would match the first number in the URL slug as
post ID, which would fail for posts with numbers in their title.

gallery_dl/extractor/patreon.py

@@ -234,12 +234,14 @@ class PatreonUserExtractor(PatreonExtractor):
 class PatreonPostExtractor(PatreonExtractor):
     """Extractor for media from a single post"""
     subcategory = "post"
-    pattern = (r"(?:https?://)?(?:www\.)?patreon\.com"
-               r"/posts/[^/?&#]*?(\d+)")
+    pattern = r"(?:https?://)?(?:www\.)?patreon\.com/posts/([^/?&#]+)"
     test = (
         ("https://www.patreon.com/posts/precious-metal-23563293", {
             "count": 4,
         }),
+        ("https://www.patreon.com/posts/er1-28201153", {
+            "count": 1,
+        }),
         ("https://www.patreon.com/posts/not-found-123", {
             "exception": exception.NotFoundError,
         }),
@@ -247,10 +249,10 @@ class PatreonPostExtractor(PatreonExtractor):
 
     def __init__(self, match):
         PatreonExtractor.__init__(self, match)
-        self.post_id = match.group(1)
+        self.slug = match.group(1)
 
     def posts(self):
-        url = "{}/posts/{}".format(self.root, self.post_id)
+        url = "{}/posts/{}".format(self.root, self.slug)
         page = self.request(url, notfound="post").text
         data = text.extract(page, "window.patreon.bootstrap,", "\n});")[0]
         post = json.loads(data + "}")["post"]