Ppolicy overlay password checking module

How about having openldap-2.4 checking whether the changed password is digit-only or alpha-only and than rejecting it?

It could be done with the non-standard password checking module openldap extension, which requires to create a C file with your rules, compile as a shared object and put in a well-known location.

I wrote a step-by-step recipe with a template C file at:

https://github.com/francescm/ppolicy-check-password