with t(s) as (select * from table(sys.odcivarchar2list('a','b',null)))
select 
  ss,
  case 
     when     regexp_like(ss, '1?', 'i') then 'true' 
     when not regexp_like(ss, '1?', 'i') then 'false' 
     else 'undef' 
  end re 
from (
    select nvl(s,chr(0)) ss
    from t
);

SS         RE
---------- -----
a          true
b          true
           true

with t(s) as (select * from table(sys.odcivarchar2list('a','b',null)))
select s, xt_regexp.get_matches(s, '1?') RE from t;

S          RE
---------- -----------------------------------
a          VARCHAR2_TABLE('', '')
b          VARCHAR2_TABLE('', '')
           VARCHAR2_TABLE('')

3 rows selected.

with t(s) as (select * from table(sys.odcivarchar2list('a','b',null)))
select s, cardinality(xt_regexp.get_matches(s, '1?')) cnt from t;

S                 CNT
---------- ----------
a                   2
b                   2
                    1

3 rows selected.

with t(s) as (select * from table(sys.odcivarchar2list('a','b',null)))
select s, cardinality(xt_regexp.get_matches(s, '^$')) cnt from t;

S                 CNT
---------- ----------
a                   0
b                   0
                    1

3 rows selected.