A character constant is a string of printable ASCII characters
  enclosed by delimiters.  It takes one of the following forms:

    [k_]'[c...]' [C]
    [k_]"[c...]" [C]

    k  Is an optional kind type parameter (1 is the default).
       It must be followed by an underscore.

    c  Is an ASCII character.

    C  Is a C string specifier.

  If no kind type parameter is specified, the type is default
  character.

  The length of the character constant is the number of characters
  between the delimiters.  In the apostrophe format, two consecutive
  apostrophes represent a single apostrophe.  In the quotation mark
  format, two consecutive quotation marks represent a single
  quotation mark.

  The length of a character constant must be in the range 0 to 2000.