Give regular expressions generating the following languages. In all cases, the alphabet is $\{$a$,$ b$\}$:

a$)\{$w$|$w begins with b and ends with 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 strings except the empty string