[youtube] Regular expression for finding next page of search results is outdated.

[RedpointsBots]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2020.07.28
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'ytsearchdate50:the']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2020.07.28
[debug] Python version 3.7.5 (CPython) - Linux-4.15.0-112-generic-x86_64-with-Ubuntu-18.04-bionic
[debug] exe versions: none
[debug] Proxy map: {}
[youtube:search:date] query "the": Downloading page 1
[download] Downloading playlist: the
[youtube:search:date] playlist the: Collected 20 video ids (downloading 20 of them)
[download] Downloading video 1 of 20

Description

The regular expression used to find the next page link in the results is currently not working. As a result, regardless of how many results you specify there is a limit to 20 results (the number of results on the first page).

The regex, found here should be:
r'href="(/results\?[^"]*\b(?:sp=[^"]+)?)"[^>]*>\s*<span[^>]+class="[^"]*\byt-uix-button-content\b[^"]*"[^>]*>Next'. I left in the incorrect part as optional, for any backwards compatibility issues.

[RedpointsBots]

I have created a pull request for this issue: #26163.