with tbl as (
              select 'N' tag,'Peter' val from dual union all
              select 'L' tag,'Borisov' val from dual union all
              select 'C' tag,'10' val from dual union all
              select 'Z' tag,'20' val from dual
            ),
   input as (
               select 'Z+C' expr from dual union all
               select 'N||L' expr from dual
              )
select  expr,
        xmlcast(
                xmlquery(
                         '/ROWSET/ROW/C'
                         passing xmltype(
                                         dbms_xmlgen.getxml(
                                                            'select ' || regexp_replace(
                                                                                        expr,
                                                                                        '([A-Z]+)',
                                                                                        '(select val from tbl where tag = ''\1'')'
                                                                                       ) || ' c from dual'
                                                           )
                                        )
                         returning content
                        )
                as varchar2(30)
               ) res
  from  input
/

EXPR RES
---- ------------------------------
Z+C  30
N||L PeterBorisov

SQL>