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
!2
But 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.