A symbol is one or more characters chosen from the set of all
letters (both upper and lower case), digits and the three characters
‘_.$’.
On most machines, you can also use $ in symbol names; exceptions
are noted in Machine Dependencies.
No symbol may begin with a digit. Case is significant.
There is no length limit; all characters are significant. Multibyte characters
are supported. Symbols are delimited by characters not in that set, or by the
beginning of a file (since the source program must end with a newline, the end
of a file is not a possible symbol delimiter). See Symbols.
Symbol names may also be enclosed in double quote " characters. In such
cases any characters are allowed, except for the NUL character. If a double
quote character is to be included in the symbol name it must be preceeded by a
backslash \ character.