Though Confluence alone does not provide password complexity controls out-of-the-box, you can integrate Confluence with your LDAP or Active Directory and configure password controls in your external directory.

Atlassian's own unified identity management and single sign-on application, Crowd, also provides password pattern/requirement checks for its configured internal directories.

Further, you may wish to take a look at Confluence Security Overview and Advisories, which answers a number of questions that commonly arise when customers ask us about application-level security for Confluence.

Another possibility is to use Enterprise Password Policy addon that offers a variety of nice password management features. Please note that it's a paid addon.