select .. case ... regexp_like in sql statement

where is wrong with this statement:
select username
          case
                 when regexp_like(USERNAME, '^[0-9]')
                 then 'select ''alter user '' || username || '' identified by values '' || chr(39) || password || chr(39) || '';''
                when regexp_like(USERNAME, '^[A-Za-z]')
                then 'select ''alter user '' || username || '' identified by values '' || chr(39) || password || chr(39) || '';''
        end
from dba_users;


I get:

ora-00905 missing keyword    at  

when regexp_like(USERNAME, '^[A-Za-z]')
                                                    *

is there a way to make this better?

thanks.