23.1.2 Effect of Readtable Case on the Lisp Reader

The readtable case of the current readtable affects the Lisp reader in the following ways:

:upcase

When the readtable case is :upcase, unescaped constituent characters are converted to uppercase, as specified in Reader Algorithm.

:downcase

When the readtable case is :downcase, unescaped constituent characters are converted to lowercase.

:preserve

When the readtable case is :preserve, the case of all characters remains unchanged.

:invert

When the readtable case is :invert, then if all of the unescaped letters in the extended token are of the same case, those (unescaped) letters are converted to the opposite case.