[youtube] Restricter DASH signature pattern

A problematic DASH url is:
https://manifest.googlevideo.com/api/manifest/dash/mm/35/key/yt5/ip/140.112.247.145/ms/pm/mv/s/mt/1433794435/id/o-AD2Od_dsOlAUYPu03ZsVWKSbGEbCJJrMp9vnXGhnyRhd/mn/sn-aigllm7r/sparams/as%2Chfr%2Cid%2Cip%2Cipbits%2Citag%2Cmm%2Cmn%2Cms%2Cmv%2Cnh%2Cpl%2Cplayback_host%2Crequiressl%2Csource%2Cexpire/fexp/9406009%2C9406821%2C9407575%2C9408142%2C9408420%2C9408710%2C9409121%2C9409208%2C9412514%2C9412780%2C9413208%2C9413426%2C9413476%2C9413503%2C9415304%2C9415753/upn/viDQrs8SnmE/as/fmp4_audio_clear%2Cwebm_audio_clear%2Cfmp4_sd_hd_clear%2Cwebm_sd_hd_clear%2Cwebm2_sd_hd_clear/playback_host/r4---sn-aigllm7r.googlevideo.com/ipbits/0/requiressl/yes/pl/20/itag/0/source/youtube/expire/1433824806/nh/EAQ/signature/81ABE6391E351BA495F5B041B00FF1257A353318.1A6E48ABB74E8F4AE73CA2CB1F963FC34E33DEE7/sver/3/hfr/1

youtube_dl/extractor/youtube.py

@@ -785,7 +785,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
             s = mobj.group(1)
             dec_s = self._decrypt_signature(s, video_id, player_url, age_gate)
             return '/signature/%s' % dec_s
-        dash_manifest_url = re.sub(r'/s/([\w\.]+)', decrypt_sig, dash_manifest_url)
+        dash_manifest_url = re.sub(r'/s/([a-fA-F0-9\.]+)', decrypt_sig, dash_manifest_url)
         dash_doc = self._download_xml(
             dash_manifest_url, video_id,
             note='Downloading DASH manifest',