'''); the string shall follow,
starting in column 12, followed by a closing single quote
(also hexadecimal code 27) that should not occur before column 20 and
must occur in or before column 80. The character string
shall be composed only of ASCII text.
A single quote is represented within a string as two successive
single quotes, e.g., O'HARA = 'O''HARA'. Leading blanks are
significant; trailing blanks are not.
Free format character strings follow the same rules as fixed format character strings except that the starting and closing single quote characters may occur anywhere within columns 11-80. Any columns preceding the starting quote character and after column 10 must contain the space character.
Note that there is a subtle distinction between the following 3 keywords:
KEYWORD1= '' / null string keyword KEYWORD2= ' ' / blank keyword KEYWORD3= / undefined keyword
The value of KEYWORD1 is a null, or zero length string whereas the value of the KEYWORD2 is a blank string (nominally a single blank character because the first blank in the string is significant, but trailing blanks are not). The value of KEYWORD3 is undefined and has an indeterminate datatype as well, except in cases where the data type of the specified keyword is explicitly defined in this standard.
The maximum allowed length of a keyword string is 68 characters (with the opening and closing quote characters in columns 11 and 80, respectively). In general, no length limit less than 68 is implied for character-valued keywords.