perl -MYAPE::Regex::Explain -e 'print YAPE::Regex::Explain->new(qr/(aaaaa(?:(?:(?!aaaaa).)*))/s)->explain'

##</code><code>##

The regular expression:

(?s-imx:(aaaaa(?:(?:(?!aaaaa).)*)))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?s-imx:                 group, but do not capture (with . matching
                         \n) (case-sensitive) (with ^ and $ matching
                         normally) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    aaaaa                    'aaaaa'
----------------------------------------------------------------------
    (?:                      group, but do not capture:
----------------------------------------------------------------------
      (?:                      group, but do not capture (0 or more
                               times (matching the most amount
                               possible)):
----------------------------------------------------------------------
        (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
          aaaaa                    'aaaaa'
----------------------------------------------------------------------
        )                        end of look-ahead
----------------------------------------------------------------------
        .                        any character
----------------------------------------------------------------------
      )*                       end of grouping
----------------------------------------------------------------------
    )                        end of grouping
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------