recursively enumerable language

regular language

context-sensitive language

context-free language

pigeon-hole principle

divide-and-conquer technique

recursion

iteration

is always regular

is always context free

both (a) and (b)

need not be regular

(a + b ) * ab

ab (a + b ) *

a ( a + b ) * b

b (a + b ) * a

product

union

complementation

kleen star