URL, LOCAL_URL and RELATIVE_URL tokens' regexes for the custom BBCodes don't have 'u' modifier which causes warning thrown when you try to post such custom BBCode:

[phpBB Debug] PHP Warning: in file [ROOT]/includes/message_parser.php on line 71: preg_match(): Compilation failed: character value in /x{...} sequence is too large at offset 63

Custom BBCodes added earlier than IDN has been applied are unaffected.