Matches an @handle, a local @name or a remote user's @name@host (HANDLE_SOURCE), with the handle as its first group. The @ must not follow a word character, so an email address mentions no one. Nor may it follow [ or (, which leaves out a Markdown link's text or URL starting with @, and also a plain (@name). The editor's suggestions match its source at the caret; the rest goes through mentionedUsernames. Markdown links mentions with a looser pattern of its own, on text already parsed, where code and URLs are out of the way.