m{
  foo
  (             # save to $1
    [^b]*       # 0 or more non-b characters
    (?:
      b+        # 1 or more b's
      (?:
        a(?!r)  # an 'a' NOT followed by an 'r'
        |       # or
        [^a]    # a non-a character
      )
      [^b]*     # 0 or more non-b characters
    )*          # that whole b... subset, 0 or more times
  )
  foo           # and foo
}x;