The regular expression:

(?-imsx:@{[s".."."gr]}\K$")

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):
----------------------------------------------------------------------
  @{                       '@{'
----------------------------------------------------------------------
  [s".."."gr]              any character of: 's', '"', '.', '.', '"',
                           '.', '"', 'g', 'r'
----------------------------------------------------------------------
  }                        '}'
----------------------------------------------------------------------
  \K                       'K'
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  "                        '"'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------