How can we filter channel entries like below:
{exp:channel:entries category="1&2&3&(5|6)"}
Is it possible to use '&' and '|' together?
Nested conditions isn't possible natively. Dandy Cat, a free plugin, will give you this exact functionality.
So for your example, you would do:
{exp:dandy_cat:entries categories=”(1&2&3)&(5|6)”}
....
{/exp:dandy_cat:entries]