Bash extract after substring and before substring

Say I have a string:

random text before authentication_token = 'pYWastSemJrMqwJycZPZ', gravatar_hash = 'd74a97f

I want a shell command to extract everything after "authentication_token = '" and before the next '.

So basically, I want to return pYWastSemJrMqwJycZPZ.

How do I do this?