TeacherTube.com broken

[DuBistKomisch]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2018.01.21. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2018.01.21

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

youtube-dl -v http://www.teachertube.com/video/multiplying-binomials-349694
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'http://www.teachertube.com/video/multiplying-binomials-349694']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.01.21
[debug] Python version 3.6.4 (CPython) - Linux-4.14.14-1-ARCH-x86_64-with-arch
[debug] exe versions: ffmpeg 3.4.1, ffprobe 3.4.1, rtmpdump 2.4
[debug] Proxy map: {}
[teachertube] 349694: Downloading webpage
ERROR: Unable to extract thumbnail; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 784, in extract_info
    ie_result = ie.extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/common.py", line 438, in extract
    ie_result = self._real_extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/teachertube.py", line 90, in _real_extract
    'thumbnail': self._html_search_regex(r'\'image\'\s*:\s*["\']([^"\']+)["\']', webpage, 'thumbnail'),
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/common.py", line 803, in _html_search_regex
    res = self._search_regex(pattern, string, name, default, fatal, flags, group)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/common.py", line 794, in _search_regex
    raise RegexNotFoundError('Unable to extract %s' % _name)
youtube_dl.utils.RegexNotFoundError: Unable to extract thumbnail; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.

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If the purpose of this issue is a site support request please provide all kinds of example URLs support for which should be included (replace following example URLs by yours):

Go to teachertube.com and pick any video, they all have the same issue.

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Description of your issue, suggested solution and other information

The teachertube extractor fails, I assume because it hasn't had any non-trivial updates since 2014 and they've probably updated the site since then. From the error it seems like a simple case of not failing if the thumbnail isn't found, but I don't know enough about the codebase to be able to fix it.

[DuBistKomisch]

@dstftw Just tried from master and it works, thanks for fixing this so quickly! You're a legend.