(e1) | (e2) is a regular expression denoting L1∪ L2

(e1) .(e2) is a regular expression denoting L1​.L2

φis not a regular expression

{ex} is a regular expression denoting L1*

finite state automation

2-way linear bounded automata

push down automata

both 1) 2-way linear bounded automata and 2) push down automata

context free language

context-sensitive language

regular language

recursive enumerable language

union, intersection

union, kleene closure

intersection, complement

complement, kleene closure

L (G) = φ

L(G) = an

L (G) = a*

L (G) = anban