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  ANSI Common Lisp   13 Characters   13.2 Dictionary of Characters

13.2.14 char-upcase, char-downcase Function

char-upcase character corresponding-character
char-downcase character corresponding-character

Arguments and Values:
character, corresponding-character - a character.

If character is a lowercase character, char-upcase returns the corresponding uppercase character. Otherwise, char-upcase just returns the given character.

If character is an uppercase character, char-downcase returns the corresponding lowercase character. Otherwise, char-downcase just returns the given character.

The result only ever differs from character in its code attribute; all implementation-defined attributes are preserved.

 (char-upcase #\a)  #\A
 (char-upcase #\A)  #\A
 (char-downcase #\a)  #\a
 (char-downcase #\A)  #\a
 (char-upcase #\9)  #\9
 (char-downcase #\9)  #\9
 (char-upcase #\@)  #\@
 (char-downcase #\@)  #\@
 ;; Note that this next example might run for a very long time in 
 ;; some implementations if CHAR-CODE-LIMIT happens to be very large
 ;; for that implementation.
 (dotimes (code char-code-limit)
   (let ((char (code-char code)))
     (when char
       (unless (cond ((upper-case-p char) (char= (char-upcase (char-downcase char)) char))
                     ((lower-case-p char) (char= (char-downcase (char-upcase char)) char))
                     (t (and (char= (char-upcase (char-downcase char)) char)
                             (char= (char-downcase (char-upcase char)) char))))
         (return char)))))

Exceptional Situations:
Should signal an error of type type-error if character is not a character.

See Also:
upper-case-p, alpha-char-p, Section Characters With Case, Section 13.1.10 Documentation of Implementation-Defined Scripts

If the corresponding-char is different than character, then both the character and the corresponding-char have case.

Since char-equal ignores the case of the characters it compares, the corresponding-character is always the same as character under char-equal.

Allegro CL Implementation Details:

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