a   include all lowercase characters [a-z]
  A   include all uppercase characters [A-Z]
  1   include the digits [0-9]
  !   include [!:$%&/()=?{[]}+*~#]
  :   the following characters upto the end of the spe-
      cification string are included in the character set.
      This way you can include any character except binary
      null (at least under unix).