$3.(20$ pts$)$ Give regular expressions generating the following languages. In all cases, the

alphabet is $\{$a$,$ b$\}$:

a$)\{$w$|$ the second symbol in w is b$,$ and the second to last symbol in w is a$\}$

b$)\{$w$|$w doesn$$t contain the substring aab$\}$

c$)\{$w$|$w contains an even number of a$$s$,$ or exactly three b$$s and any number of a$$s$\}$

d$)$ All non$-$empty strings except b