person number should be validated based on Legal entity

Summary:

Content (please ensure you mask any confidential information):

The person number generation for particular LE starts with differnt prefix and
continues with number. The number generation is manual and should not be duplicated.
Each LE has particular 3 digit prefix.
When the User enters the person number, the system should check the last number generated along with prefix for that legal entity of user
and not allow the user to enter already available person number.

We need to validate the person number and Legal entity in the autocomplete rules.
Please assist us with the way how we can achieve this and