Search notes:

ORA-00911: invalid character

Although the dollar sign, the underscore and numbers are permitted in identifiers, identifiers cannot start with these characters (except when quoted):

select _leading_underscore from dual;

String literals need to be quoted:

create table tq84_xyz(
  col varchar2(20)
);

insert into tq84_xyz values (#);

drop table tq84_xyz;

ORA-00911: invalid character

See also