My company, which specializes in logistics and transportation, delegated the majority of the backend microservices to our team. All of the microservices (which our team inherited from "past generations") appear to have inconsistent input field validation. Mobile number format validation is a good example. There would undoubtedly be a need to add other validations, such as prefixes and other mobile number lengths to take into account, if our clientele expanded to other nations.
Also, not all microservices read from the same databases.
How should this mobile number format validation be centralized across all microservices? The only other option that comes to mind is creating a separate microservice for this, but this seems to be an overkill considering that for now, this will only be applied to mobile number format validation. The same is true for Drools, a business rules engine. Are there other available options for smaller scale rules like this one?