Are composed of letters, and non initial digits. Also, any other character may be included in an identifier by preceding (escaping) it with a ! (exclamation mark). Identifiers are unlimited in length. Some examples of identifiers are :
a AVeryLongIdentifier U238 emsg!* !!!&x !2But 2 and u:238 are not identifiers. As an exception, the LISP reader treats an unescaped special character, other than those that are meaningful to it (i.e. parentheses, dot, quotes etc.) as a single character identifier. So u:238 is read as a sequence of three atoms: identifier u , identifier !: and integer 238 . In fact the LISP reader will make S-expressions from anything you invent for input.