YouTube profile input will not accept valid YouTube account url's

What the title says. The YouTube profile field refuses valid url's in standard YouTube formats.

Original post here: https://www.phpbb.com/community/viewtopic.php?p=15964912#p15964912

Original grumble, which led to that post, can be found here: https://talkrational.org/tikal/viewtopic.php?t=611

Url's tested were https://www.youtube.com/channel/UCa2rEXCNODl9a7gLygpQbAA

and https://www.youtube.com/@lanetaylor3900/videos

Both of these are fine if you just drop them in the browser's address bar, but that member couldn't put them on his phpBB profile.

Error message was: "The field “YouTube” has invalid characters, only alphanumeric or _,-. characters are allowed and the first character must be alphabetic."

Definitely affects 3.3.8. Have not tested on 3.3.10 yet, but have not seen anything in the bug tracker or release notes that leads me to think there have been any changes in this since 3.3.8.