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Does the following work when used in comments to refer to a specific user (say, xuser):

@xuser: blah blah blah...

Or should I just use separate punctuations?:

@xuser : blah blah blah...
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up vote 8 down vote accepted

Yes, that works. Since a colon is not a valid character in a username, the reply-to username in your example is considered to end on the "r".

Note that while a period is valid in usernames,

Thanks, @xuser. That was helpful.

will also work, because a trailing period is removed from the mention (to accomodate this precise scenario). And since July 2014, multiple trailing dots are removed too.

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Does this mean @R.. won't work? Or is it clever about that? – marcog Jan 21 '11 at 14:01
@marcog: We try to handle those edge cases as intuitively as possible. "@R.." will match a user called "R. Smith", if that's what you mean. It will also match a user called "R." (which isn't a legal new username anymore), and also matches "R..". – balpha Jan 21 '11 at 14:42
@Arjan: Note that I've rewritten a lot of the comment matching code, so take previous statements on edge case handling with a pinch of salt (in this case it's true, though). – balpha Jan 21 '11 at 14:47
The 11 points (which are in fact many more points) make a nice set for a unit test...? :-) – Arjan Jan 21 '11 at 14:49
@Arjan: Re unit tests -- there's a bunch of them in this case :) And re R. Smith: As I said, the trailing period is removed, so we're looking for "R.", which is the first name of "R. Smith", so the two-character-first-name-matching holds. By the way: I tested the algorithm on a (few months old) copy of the Meta database; except for 8 users (who have no latin letters in their usernames), a reply with "@" + username.Replace(" ", "") always matched; with and without an additional period. – balpha Jan 21 '11 at 15:01
Lesson learned: You really shouldn't try to reply to "R. Smith" with "@R.". Use "@R.Smith". – balpha Jan 21 '11 at 15:05
Note that a single quote is valid in a display name too. Hence, in @name's comment fails to notify name. (But using @O'Reilly would work of course.) – Arjan Feb 3 '11 at 10:54
@Arjan: Excellent point; I'll have a look at that. – balpha Feb 3 '11 at 10:59
No, @balpha, don't worry, it's already official ;-) – Arjan Feb 3 '11 at 11:04
@Arjan: I'm afraid you'll have to edit again :) After the next build, these genitive constructs will be handled as well. – balpha Feb 3 '11 at 12:39

You don't need to seperate any puntuation that follows the user's name.

See the FAQ question How do comment @replies work?, specifically point 12 of the answer.

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