Password should not be a part of username script

I'm creating a script that would accept user inputs in the form of a registration. Everything is all set up I just wanted to add one bit of a function here. I would like to prohibit users from using passwords that are part of their username. I would like to ask for help in formulating the script for it but here's what I have for now.

Assuming that both "username" and "password" are variables

passcheck=$(echo "*$username*")
if ($username = $passcheck)
then
echo "You can't use a password with your username in it!" 
else
echo "ok"

What i'm trying to achieve here is that as long as any part of the whole username is used as a password despite having numbers or symbols AFTER or BEFORE it would be unacceptable

Example of an invalid input:

username=gifter
password=gifter100!

Another example of an invalid input:

username=doctor
password=thegooddoctor100!

Example of a valid input:

username=starbucks
password=star100bucks!