/^([-_\p{N}\p{L}])+$/iu
because i don't like how english-centric only allowing ascii letters/numbers is though this only applies to local users as of now, currently all nicknames are tolerated when parsing remote feeds and i just do mentions how yarn does (just the feed url)in the wild, i've noticed a texedus feed with spaces in the nick (where its spec explicitly disallows whitespace in the nick) and feeds with other symbols in the nick too. honestly, i think we should just tolerate arbitrary nicknames for sake of user expression (while stripping or converting unreasonable characters) and just leave them out of mentions