The following regex will validate the top-level domain and whether it has http(s):// or not:

^(http(s)*://)*[a-z0-9.-]+$

Plug it to regexp_like and you get:

with urls as (
  select 'www.oracle.com' u from dual union all
  select 'http://www.oracle.com' u from dual union all
  select 'https://www.oracle.com' u from dual union all
  select 'https://www.oracle-here.com' u from dual union all
 select 'https://www.####.com' u from dual union all
  select 'not an URL' u from dual union all
  select 'http://' u from dual 
)
  select * from urls
  where  regexp_like(u, '^(http(s)*://)*[a-z0-9.-]+$');

U                         
---------------------------
www.oracle.com             
http://www.oracle.com      
https://www.oracle.com     
https://www.oracle-here.com

But this is basic. It only checks the top-level domain. Checking that a string is a valid URL is complicated:

http://stackoverflow.com/a/190405/1485955

Unless there's a pressing need to ensure that these are real URLs, writing a regular expression is probably more hassle than it's worth.