As it is set up now, the program assumes that the input file is coded using 8-bit ISO 8859-1 code, also known as Latin-1 character set, unless it is compiled for MS-DOS, in which case it uses the character set of the IBM-PC. (GNU ptx is not known to work on smaller MS-DOS machines anymore.) Compared to 7-bit ASCII, the set of characters which are letters is different; this alters the behavior of regular expression matching. Thus, the default regular expression for a keyword allows foreign or diacriticized letters. Keyword sorting, however, is still crude; it obeys the underlying character set ordering quite blindly.