I wanted to share a great tool for coming up with a regular expression for password complexity requirements (which you can customize in Admin Tools>General Settings>Global Attributes>Password Regular Expression):  https://regex101.com/   The site has a tester, and will help you write the expression you need.

We are currently using using the expression (?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{6,}  which is at least one lowercase letter, at least one uppercase letter, at least one number, and a minium of 6 characters.

See http://www.rockrms.com/Rock/BookContent/9/9#generalsettings for more details about passwords.  Hope this is helpful to someone!