it works for every case except one of the main test cases

What if there is nothing to print?

This sounds somewhat unusual. If you haven't read the FAQ item on printing the test case input then please do, paying special attention to flushing, and order the prints are displayed w.r.t. assertion message (i.e. printed output is present above failure message).

If you already know all of this and you still cannot pass, show us your code and we will take a look.

If you want to see the input, you have to print the input, not the output...

Valid passwords will only be alphanumeric characters

There's a small mistake in your regex.

Your code is wrong (use print to see what you're doing wrong), and please use markdown formatting and mark your post as having spoiler content when posting code like that. Btw, use Question label when the problem is your code, not the kata.