(Corresponds to Section 4.8 of ANSI X3.9-1978 FORTRAN 77.)
A character constant may be delimited by a pair of double quotes (`"') instead of apostrophes. In this case, an apostrophe within the constant represents a single apostrophe, while a double quote is represented in the source text of the constant by two consecutive double quotes with no intervening spaces.
A character constant may be empty (have a length of zero).
A character constant may include a substring specification, The value of such a constant is the value of the substring—for example, the value of `'hello'(3:5)' is the same as the value of `'llo''.