REGEXP_LIKE query's working differently.

Question

Hi,
Version :Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
i didn't understand why below query's working differently.
First one:
                 select 'TRUE' from dual where REGEXP_LIKE( 'a', '[:alnum:]');
'TRUE'
TRUE  
       Second one:
                  select 'TRUE' from dual where REGEXP_LIKE( 't', '[:alnum:]');
no rows selected
       Please clarify.

Answer 1

Check this...
WITH xx AS (
    SELECT 'a' AS lvl FROM DUAL union all
    SELECT 'l' AS lvl FROM DUAL union all
    SELECT 'n' AS lvl FROM DUAL union all
    SELECT 'u' AS lvl FROM DUAL union all
    SELECT 'm' AS lvl FROM DUAL union all
    SELECT 'r' AS lvl FROM DUAL union all
    SELECT 's' AS lvl FROM DUAL union all
    SELECT 't' AS lvl FROM DUAL union all
    SELECT 'w' AS lvl FROM DUAL
select 'TRUE',x.lvl from xx x where REGEXP_LIKE(x.lvl, '[:alnum:]');Any other character than {a,l,n,u,m}, will return FALSE.
HTH
Ranit B.