D:\>perl -MYAPE::Regex::Explain -e"print YAPE::Regex::Explain->new(shift)->explain" "/((?:(?:[^\n@]+|@[^@]*@)\n?)+)/gs"
The regular expression:

(?-imsx:/((?:(?:[^\n@]+|@[^@]*@)\n?)+)/gs)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    (?:                      group, but do not capture (1 or more
                             times (matching the most amount
                             possible)):
----------------------------------------------------------------------
      (?:                      group, but do not capture:
----------------------------------------------------------------------
        [^\n@]+                  any character except: '\n'
                                 (newline), '@' (1 or more times
                                 (matching the most amount possible))
----------------------------------------------------------------------
       |                        OR
----------------------------------------------------------------------
        @                        '@'
----------------------------------------------------------------------
        [^@]*                    any character except: '@' (0 or more
                                 times (matching the most amount
                                 possible))
----------------------------------------------------------------------
        @                        '@'
----------------------------------------------------------------------
      )                        end of grouping
----------------------------------------------------------------------
      \n?                      '\n' (newline) (optional (matching the
                               most amount possible))
----------------------------------------------------------------------
    )+                       end of grouping
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  /gs                      '/gs'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

D:\>