What are the contraction rules for noun such as Bob, tickets, concert, etc?

Ok, Here is the tricky ones.

Googling "The concert's sold out" returns no result, whereas googling "The concert was sold out" returns a lot of result.

Also, I don't see anyone say "the tickets're sold out" but most people say "the tickets were sold out", so "were" in this case can not be contracted.

Also, "Bob is a dog" could become "Bob's a dog" but then people may confused because it could mean "Bob has a dog".

What are the contraction rules for noun such as Bob, tickets, concert, etc?