unread-char character &optional input-stream => nil
Arguments and Values:
character---a character; must be the last character that was read from input-stream.
input-stream---an input stream designator. The default is standard input.
unread-char places character back onto the front of input-stream so that it will again be the next character in input-stream.
When input-stream is an echo stream, no attempt is made to undo any echoing of the character that might already have been done on input-stream. However, characters placed on input-stream by unread-char are marked in such a way as to inhibit later re-echo by read-char.
It is an error to invoke unread-char twice consecutively on the same stream without an intervening call to read-char (or some other input operation which implicitly reads characters) on that stream.
Invoking peek-char or read-char commits all previous characters. The consequences of invoking unread-char on any character preceding that which is returned by peek-char (including those passed over by peek-char that has a non-nil peek-type) are unspecified. In particular, the consequences of invoking unread-char after peek-char are unspecified.
(with-input-from-string (is "0123") (dotimes (i 6) (let ((c (read-char is))) (if (evenp i) (format t "~&~S ~S~%" i c) (unread-char c is))))) >> 0 #\0 >> 2 #\1 >> 4 #\2 => NIL
Exceptional Situations: None.
peek-char, read-char, Section 21.1 (Stream Concepts)
unread-char is intended to be an efficient mechanism for allowing the Lisp reader and other parsers to perform one-character lookahead in input-stream.