# * <tt>:in</tt> - A synonym(or alias) for <tt>:within</tt>.
# * <tt>:allow_nil</tt> - Attribute may be +nil+; skip validation.
# * <tt>:allow_blank</tt> - Attribute may be blank; skip validation.
#
# * <tt>:too_long</tt> - The error message if the attribute goes over the maximum (default is: "is too long (maximum is %d characters)").
# * <tt>:too_short</tt> - The error message if the attribute goes under the minimum (default is: "is too short (min is %d characters)").
# * <tt>:wrong_length</tt> - The error message if using the <tt>:is</tt> method and the attribute is the wrong size (default is: "is the wrong length (should be %d characters)").