symbol_character =
  ? not (")" | "(" | "[" | "]" | U+0022 | \p{Separator}) ? ;

symbol =
  symbol_character , { symbol_character } ;

quoted_character =
  ? not U+0022 ? ;

quoted_string =
  (quoted_character | escape) , { (quoted_character | escape) } ;

escape =
    escape_carriage
  | escape_newline
  | escape_tab
  | escape_quote
  | escape_unicode4
  | escape_unicode8 ;

escape_carriage =
  "\r" ;

escape_newline =
  "\n" ;

escape_quote =
  "\" , U+0022 ;

escape_tab =
  "\t" ;

escape_unicode4 =
  "\u" ,
  hex_digit , hex_digit , hex_digit , hex_digit ;

escape_unicode8 =
  "\u" ,
  hex_digit , hex_digit , hex_digit , hex_digit ,
  hex_digit , hex_digit , hex_digit , hex_digit ;

hex_digit =
  "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "0" |
  "a" | "A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "E" | "f" | "F" ;

expression =
    symbol
  | quoted_string
  | "[" , { expression } , "]"
  | "(" , { expression } , ")" ;
