Too add to what Akira mentioned, please keep in mind that advanced registration forms are a paid feature (developer plan or higher) within FusionAuth. Within this feature set, you can use regex commands and add your own custom validation as shown below.
ba3d6675-f745-4ace-9ce5-2c633c2a5f52-image.png
Additionally, under tenants > password there are a number of custom password rules that you can set as well.
In addition to the tenant, there are also some password rules that you can configure on the application level under applications > registration > self-service registration > enabled > type: basic . Here is where you will find a confirm password toggle as well.
Finally, the above applies to a POST event. Meaning the form has to be submitted before feedback is displayed to the user indicating that they failed validation.
There are some Github issues that we are tracking for more realtime validation like events as well. If you have a unique use case - feel free to log an issue as well.
https://github.com/FusionAuth/fusionauth-issues/issues/1223
Thanks
Josh