Humm, trying to create a regex filter in my mastodon timeline ot only show me Russian language posts -- the easiest way, I guess, is to use a regex matching Russian Cyrillic, like [АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯабвгдеёжзийклмнопрстуфхцчшщъыьэюя]+ or some such.
But that does not work. Are #Mastodon regexes special somehow?..