Regular expressions: Let $=\{a,b)$ be defined as the alphabet. Write down regular

expressions for $($a$)$ through $($d$).$ Further, write down some of the strings which fall under

each of the below four languages:

$($a$)L_1$ is a language which defines all strings ending in the string $"$ab$".$

$($b$)L_2$ is a language which defines all strings containing "aba"

$($c$)L_3$ is a language which defines all strings not ending in the string "aba"

$($d$)L_4$ is a language which defines all strings beginning with $"$aa$"$ and ending in $"$bb$"$