Query:
select table_name,column_name

from (select rownum,table_name, regexp_substr(dbms_xmlgen.getxml('select * from "'||table_name||'"'),'<[^>]*>&string</[^<]*>') column_name from user_tables)

where length(column_name)!=0;

Error:
ORA-31011: XML parsing failed
ORA-06512: at "SYS.DBMS_XMLGEN", line 176
ORA-06512: at line 1
31011. 00000 - "XML parsing failed"
*Cause: XML parser returned an error while trying to parse the document.
*Action: Check if the document to be parsed is valid.

SQL> set serverout on
SQL> declare
  2    cursor c is
  3     select rownum,
  4            table_name,
  5              dbms_xmlgen.getxml('select * from "'||table_name||'"') column_name
  6     from user_tables ;
  7    r c%rowtype;
  8  begin
  9    open c;
 10    loop
 11      fetch c into r;
 12      exit when c%notfound;
 13      dbms_output.put_line(r.table_name);
 14    end loop;
 15  end;
 16  /
DEPT
EMP

PL/SQL procedure successfully completed.