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.

1  –  C Strings

  String values in the C language are terminated with null characters
  (CHAR(0)) and can contain nonprintable characters (such as a
  backspace).

  Nonprintable characters are specified by escape sequences.  An
  escape sequence is denoted by using the backslash (\) as an escape
  character, followed by a single character indicating the
  nonprintable character desired.

  This type of string is specified by using a standard string
  constant followed by the character C.  The standard string constant
  is then interpreted as a C-language constant.  Backslashes are
  treated as escapes, and a null character is automatically appended
  to the end of the string (even if the string already ends in a null
  character).

  The following C-style escape sequences are allowed in character
  constants:

     Escape Sequence      Represents
     ---------------      ----------
     \a or \A             A bell
     \b or \B             A backspace
     \f or \F             A formfeed
     \n or \N             A new line
     \r or \R             A carriage return
     \t or \T             A horizontal tab
     \v or \V             A vertical tab
     \x"hh" or \X"hh"     A hexadecimal bit pattern
     \"ooo"               An octal bit pattern
     \0                   A null character
     \\                   A backslash

  If a character constant contains any other escape sequence, the
  backslash is ignored.

  A C string must also be a valid Fortran string.  If the string is
  delimited by apostrophes, apostrophes in the string itself must be
  represented by two consecutive apostrophes ('').

  For example, the escape sequence \'string causes a compiler error
  because Fortran interprets the apostrophe as the end of the string.
  The correct form is \''string.

  If the string is delimited by quotation marks, quotation marks in
  the string itself must be represented by two consecutive quotation
  marks ("").

  The sequences \"ooo" and \x"hh" allow any ASCII character to be
  given as a one- to three-digit octal or a one- to two-digit
  hexadecimal character code.  Each octal digit must be in the range
  0 to 7, and each hexadecimal digit must be in the range 0 to F.
  For example, the C strings '\010'C and '\x08'C) both represent a
  backspace character followed by a null character.

  The C string '\\abcd'C) is equivalent to the string '\abcd' with a
  null character appended.  The string ''C represents the ASCII null
  character.