Q:

Oracle regular expression regexp_like

I have simple question. 
why would :
select regexp_like( (select 'https://www.hotmail.com' from dual) ,'(f|ht)tps?:')  from dual ;

not work? 

A:

What do you expect this query to do? 
regexp_like is a regular expression version of the LIKE statement so it makes sense to use it in the same sorts of places that you would use a LIKE.   You wouldn't try to directly SELECT the result of a LIKE statement.   You could, however, embed the regexp_like in a case statement.   For example
SQL> ed
Wrote file afiedt.buf

  1  select (case when regexp_like( 'https://www.hotmail.com' ,'(f|ht)tps?:') 
  2               then 'true'
  3               else 'false'
  4           end) does_it_match
  5*   from dual
SQL> /

DOES_
-----
true

regexp_like is a function that returns a boolean.   Oracle SQL, however, does not support the boolean data type so you cannot directly SELECT the result of the function just like you couldn't SELECT a function you write that returns a boolean. 