regular expressions:

Let L$($r$1)$ where r$1=($a $+$ b$)"$ with alphabet la$,$ b$\}$

and let L$($r$2)$ where r$2=($c $+$ d$)*$ with alphabet $\{$ c$,$ d$\}$

then L$($r$1).$L$($r$2)$ on the alphabet $($a$,$b$,$c$,$d$\}$ is a language that represents any string containing zero or more occurrences of a or b or c or d$.$