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As m4 reads its input, it separates it into tokens. A
token is either a name, a quoted string, or any single character, that
is not a part of either a name or a string. Input to m4 can also
contain comments. GNU m4 does not yet understand
multibyte locales; all operations are byte-oriented rather than
character-oriented (although if your locale uses a single byte
encoding, such as ISO-8859-1, you will not notice a difference).
However, m4 is eight-bit clean, so you can
use non-ASCII characters in quoted strings (see Changing the quote characters),
comments (see Changing the comment delimiters), and macro names (see Indirect call of macros), with the
exception of the NUL character (the zero byte ‘'\0'’).