how do I accomodate multiple spellings in linux if/then script?

I am new to coding and my Linux/Ubuntu assigment is requesting that I accomodate mulitple spellings for the argument in an if/then statement. I can't find anything about this online, at least nothing with these search terms. Can someone help? I am using 15 workstation.

1. Write a program called isyes that returns an exit status of 0 if its argument is yes, and 1 otherwise. For purposes of this exercise, consider y, yes, Yes, YES, and Y all to be valid yes arguments

2. Is there a way to so thais that will not reveal all the different spellings in either the question or the response? That is ugly.

Thank you!