So, according to this post and practical experience it's not possible to notify more than one user. For example:

@alice and @bob, though I agree with your general concern, ...

is blocked by the system.

The only reasons I have heard about were (see comments):

  • Spam control? (doesn't make any sense to me, but that's what the post says)
  • Speed? (Regex needs to match only first mention)
  • Technical limitations? (Comment on question: System can't handle sending multiple notifications...)

All these supposed reasons sounds like a bad idea to me, so I doubt those are the real reasons. But I would love finding out what the rationale is in that case.

  • 1
    Comments are not for extended discussion. Preventing pinging lots of people helps keep away from extended discussions if nothing else.
    – Flexo
    Apr 23 '15 at 23:58
  • @Flexo By that logic removing the entire notification feature would be even better right? If anything addressing both users with the same comment will streamline the discussion instead of keeping it as two separate discussions about the exact same thing. Apr 24 '15 at 1:28
  • 1
    @DavidMulder How is removing the entire notification feature relevant to what Flexo stated? One ping is fine, many is not, and there is no logic in removing the ability to have one ping just because "many" would be bad. Comments are for simple/basic things, as Flexo stated, and neither I nor Flexo are making this up - See the official notes for yourself: meta.stackexchange.com/help/privileges/comment - "Comments are temporary 'Post-It' notes" and read "When shouldn't I comment?"
    – James
    Apr 24 '15 at 2:06

When a user comments, the question OP gets a notification, plus the user they are pinging, and potentially the answerer depending on where the comment is.
So this is already a potential of 3 users being pinged from one comment.

Also, how are the limitations a bad idea? They stop real problems from occurring.
Maybe the "titles" as you listed in your question don't do the "reasons" justice.

Spam control? (doesn't make any sense to me, but that's what the post says)

Imagine a comment:

[opens-long-coat] Psst, wanna buy some viagra?? - @James, @DavidMulder, @You, @Them, @someoneelse, @bob, @sally, @foobar

This would also go to the OP and potentially the answerer too.
So this could be used for spam purposes, so not allowing it "stops potential spam".

And I take "spam" as also "unwanted" and "bothersome".

Imagine a user pinging 3, 4 or more other users to say "Thanks" or "help me".
This needs to be avoided too.

Speed? (Regex needs to match only first mention)

I'm not sure what you mean by this.
If regex was used to match only the first mention, then you still wouldn't have multiple notifications.
In terms of being able to work around the limitations, and use @ for multiple users and only ping one, see the bottom of my answer.

The "speed" issue will likely take into account the fact there is constantly a lot of things occurring around the sites - questions, answers, comments, reviews, flags, votes, etc.

Even comment notifications alone:
There are 4 million users on Stack Overflow, there will be many notifications sent every minute.
All needing scripts: When sent; Save in DB; Show to pinged user(s); DB updated when user(s) read them; etc.

With everything else going on around the site, having bulky comment notifications is not a good idea.
Comments are not that important anyway, certainly not for pinging multiple users.
It's been debated to death that Stack is not social network sites, so comments and certainly pings are supposed to be a means to discuss necessary things to obtain the answer, not chat, or debate etc.

Technical limitations? (Comment on question: System can't handle sending multiple notifications...)

Not being a dev in Stack I cannot state why, but I imagine that there will be some truth in this.
And even though they might be able to work around the technical issues, considering the spam potential, speed issue, and other things, it just won't be worthwhile.

If you are having a discussion with two or more users in comments other than the question OP and answerer, then they'll likely be checking back on comments anyway.

If not, and it's very useful or important, then either start a temporary chat room between you all, or in rare and very useful/important situations just use two comments to ping both users.

You stated "Regex needs to match only first mention".
This can already be worked around by the users anyway, by using the backtick.

How do comment @replies work?

Comments containing more than one @name are blocked unless they contain a backtick `. In the latter case, only the first name mentioned using the @name syntax will be notified. For example, @alice `@bob Hi!` will notify Alice (if she has participated in that post), but not Bob.

An exception is the case when the first @name either matched nobody, or matched the post's author (and thus isn't necessary); in this case, the next @name will be checked.

  • Ehm, did you bother clicking the link for the source of those speculations? It was just random speculation of users in comments. Regarding spam: this is handled on SE using reputation based mechanisms. Regarding speed: "Plus, it probably makes the system faster, because you only have to search for the first .@user" this is a micro-micro-'optimalization' which definitely wasn't a reason for the implementation. And regarding technical reasons, there was no real grounds for this speculation, it was just how it was thus technical reasons were speculated... Apr 24 '15 at 1:27
  • If you do not want an answer for the things in your question, then edit your question and remove them. It seems this is what your question is about, so I answered about them as per your question. Regarding the list in your question, you stated they are "not likely the real reasons", so I answered to state why the reasons given have merit and can be argued as valid. My answer regarding speed was not about "micro-optimisation". My answer has some valid points, even if just for continuing the debate.
    – James
    Apr 24 '15 at 1:40
  • I will take that as a "no I indeed did not bother checking the source for what you were talking about, but I just took the liberty to give it my own baseless interpretation none the less". Either way, turns out this was a duplicate :( And thanks for the fix in the title still :) Apr 24 '15 at 2:00

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