i.e. specify ALL the invalid characters and translate each one to (say) 'X'. If after translation the string is same as before, it didn't contain any invalid characters.
Alternatively, if it is easier to specify what IS valid then:
CONSTRAINT CHECK (TRANSLATE(column_a,'#ABCDE...','#') IS NULL)
i.e. specify ALL the valid characters and translate each one to NULL (the '#' is just there so that the second string isn't empty). If the translated string is not null, it must have contained an invalid character.