with spec_alpha_chars as (
select 'DÉLL' name from dual union
select 'QUÉST' name from dual  union
select 'IBM' name from dual
 )
select * from spec_alpha_chars

--all valid characters
select regexp_replace('DÉLL','[DELL]','') from dual;
--or negate all invalid characters
select regexp_replace('DÉLL','[^É]','') from dual;
Example:
--valid
SELECT CHR(1+level) FROM dual CONNECT BY level <= 121;
--invalid
SELECT CHR(1+level) FROM dual WHERE level > 121 CONNECT BY level <= 254 ;

--replace match pattern  (convert Fails)

WITH spec_alpha_chars AS
  ( SELECT 'DÉLL' name FROM dual
  UNION
  SELECT 'QUÉST' name FROM dual
  UNION
  SELECT 'IBM' name FROM dual
  )
SELECT regexp_replace(name,'[^{|}~€‚ƒ„...†‡ˆ‰Š‹ŒŽ''""•--˜™š›œžŸ ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ]',''),
  name
FROM spec_alpha_chars
WHERE regexp_like(name,'[{|}~€‚ƒ„...†‡ˆ‰Š‹ŒŽ''""•--˜™š›œžŸ ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ]');

regexp_replace('DÉLL','[DELL]','')