[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