This is not related to the RFC. IRC clients have had to adapt over the years to cater for server implementations that go beyond the RFC. In this case, I mentioned the reason in my post:
If I recall correctly, at one time mIRC did store more than nine characters for the user string, however due to bug reports relating to that behaviour mIRC was changed to comply with the maximum length. I can't remember the reason for the original bug report unfortunately apart from the fact that there were side-effects on some IRC networks when the maximum length was not enforced.
While I could revert this change, this will result in the same issue that users reported years ago that led to this change in the first place.
1. mIRC itself allows userids of up to 10 characters on the IDENT options page. $address(nick,5) works on these, but $address(nick,6) still truncates the first character.
$mask() was made to work this way based on input from users who were having issues with servers that could not handle long userids. Anything beyond 9 characters is chopped to nine characters and prefixed with a wildcard.
2. As stated in that link, the RFC states "While the maximum length is limited to nine characters, clients SHOULD accept longer strings as they may become used in future evolutions of the protocol." and mIRC doesn't follow this.
mIRC did follow this - it originally had no limit. However, as different server implementations used different methods, and were affected in different ways by userids longer than nine characters, there was no other way to prevent these issues.
3. I found this because I use the $address(nick,5) to add addresses to the DCC trust list and DCC sends were failing - so mIRC is inconsistent in its handling of userids longer than 10 characters, using the full userid when comparing with the trust list but only the first 10 characters when providing it in $address(nick,5).
If you can provide a step by step method of reproducing this issue, so that I can see where it is being inconsistent, I will try to reproduce it here. Note that any further changes will need to chop the userid to 9 characters.
