syntactic meta language
decidable language
intermediate language
high level language
(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