How do comment @replies work?

Question

How do the comment replies (aka "pinging users") work?

• Who can I send an @reply to?
• How do I respond to a specific user when entering my comment?
• Will they be notified?
• What do people mean when they talk about "pinging" another user?

Related:

1. How do I view the recent replies to things I have written?

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Answer 1

You can use @name syntax anywhere in your comment to reply to a specific user. This will notify that user in their global inbox. There can also be notification through email if you set it up in the preferences found in your profile page. This feature is specific to comments and doesn't work in posts.

Who can be notified with this feature?

• The author of the specific post (question or answer).

  Note that the author of the post will always be notified of any new comment. You may still use it for clarity, if needed; however, if there are no comments, or only you or the author have commented on the post so far, the @name will be automatically removed from the beginning of the comment, as it adds no value. (To avoid breaking sentences, mentions not at the beginning will not be removed.)

• Any user who has a visible (non-deleted) comment on the post.

  This is usually through the @name feature. Additionally, if a user comments on their own post and there is only one other person who has previously commented on that post, then that person is also notified, even if @name is not used.

• Any user who has edited the post (does not include pending or rejected edit suggestions).

• For questions: The moderator or gold badge holder who closed or reopened the question, provided they were the only one to do so. Users who have closed or reopened a question without a binding close vote (i.e. without a gold tag badge or a moderator vote), and those who bindingly voted, but other users were also involved (e.g. closed by User1, User2, and Moderator) cannot be notified.

• For questions: any user who put a bounty on the question (current or expired)

Keep in mind that the question and answers are all considered independently. For example, if Alice was the author of the question, then you cannot notify her by commenting on Bob's answer (unless Alice also participated in that answer). Similarly, you cannot notify Bob by commenting on Alice's question (unless Bob participated in the question).

Why do some names not appear in the auto-complete box?

Only users who have commented on the post are populated into the list. Editors and other users from the post's history will never appear there, even if they are able to be notified by typing manually. The author is also not included unless they commented.

Users without an account on the site (i.e. if their name shows up without a link, such as deleted users) will not show in the autocomplete, as there's no active account to which to send the notification.

If you are using the mobile apps, there is no autocomplete feature when typing; however, you can still autocomplete users who commented by tapping on an existing comment and using the reply button at the top. (The button won't show in the above case of the commenting user not having an active account on the site.)

Can I notify more than one person at a time?

No. 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. Note that these checks only apply to the first @name, not subsequent ones; if the first one matches someone, and there are multiple @name mentions (or any text of the form @something), the comment will still be blocked.

Can I change who gets notified after the comment is posted?

When editing a comment within its limited editing period, if you change or add @name, the notification may or may not reach the new recipient depending on timing.

Are there any special keywords for notifying users?

No. Things like @op, @downvoter, or @all have no special meaning and will not trigger any notifications.

How do the names get matched?

• You must include @name, where name is a reasonable match to a user's current display name at the time the comment is submitted. You cannot use previous display names the target may have had.

• The notification must begin with a space or be at the start of the comment. For example, you cannot use markup such as italics.

  • Note that the autocomplete may still show up even if there is a non-space character at the beginning of an @name mention. This does not mean that it will work.

• If the first word in the display name is at least three characters long, then there must be a starts-with, case-insensitive match of at least three characters in the display name. This means @a and @ab will never match anyone, unless a user uses a first word that is only two characters. Like: @Jo will notify Jo Miller, but not John, and @B. will notify B. Gates, but not B.Gates. If there are more than three characters in @name, then all given characters must match (neither @alix nor @aliceinwonderland will match user Alice).

• Matching is performed in reverse chronological order, so if five people named John are participating, @john will match the most recent John. (Use the next rule to differentiate.)

• Spaces are removed from the display names for matching purposes. So to match Peter Smith you may use @pet, @peter, @peters, or @petersmith. The last two are useful if Peter Jones is also participating, who can then be distinguished using @peterj. However, no spaces are allowed in the @name itself. Like to notify P Smith, one must use @psm or @psmith. (Here @P Smith would be handled as just @P, which is too short.) Single quotes, dots, dashes and underscores should not be removed.

• Special characters are replaced with their simple equivalent. To reply to Piëre you can use both @piëre and @piere. And to reply to Jörg you could use @jorg, but not @joerg.

Does other punctuation affect the notification?

The system does attempt to ignore most punctuation to allow for proper notifications, such as a single trailing dot, comma, or colon. The exact matching rules are not documented, but extensive tests performed on 30 August 2011 indicate that the following algorithm is used:

• Take the first word immediately following the @, where the word boundary is determined by a space, a character not valid in user names (such as : , / ! ?) or the end of the comment.
• Remove one trailing dot . from the resulting string if applicable (and if the string has more than two characters).
• If the remaining string ends in ' or 's, remove that piece from the string.
• The resulting string will be matched against the user name with spaces removed.

Example: The comment ends with @O'Conner's.). The word immediately following the @ is O'Conner's. since ' and . are valid in display names. Then the trailing dot is removed, and finally the 's is removed, resulting in O'Conner.

Some examples of supported notifications:

• @name some text
• @name: some text
• @name. Some text
• @name, some text
• some text, @name
• some text, @name, more text
• Some text, @name.
• This is mentioned in @name's comment.
• @P. for both P. Smith and P. Jones (whoever commented most recently), but not P.Smith nor P.Jones. Likewise, @P. Smith is handled as @P., so also matches P. Smith or P. Jones.
• @psm or @psmith for P Smith
• @peters or @peterj for Peter Smith or Peter Jones respectively
• @name...

Examples that will not trigger notifications:

• abc@name
• *@name*
• *@name:*
• [@name](http://some-url)
• @[name](http://some-url)
• @P Smith