Any idea what aws_backup_selection “match patterns” refers to?

https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/backup_selection#resources describes the resources attribute’s value:

An array of strings that either contain Amazon Resource Names (ARNs) or match patterns of resources to assign to a backup plan…

This isn’t explained further or shown in the examples, and the upstream AWS API documentation doesn’t describe support for anything other than lists of ARNs:

https://docs.aws.amazon.com/aws-backup/latest/devguide/API_BackupSelection.html

(The service will reject use of wildcards in ARNs, too)

It’s not match as in regex/wildcards. It’s that you can define a pattern of resources (using tags and conditional) to match certain resource with those tags. So you don’t have to specific each resource, but you can tell terraform which resources to dynamically match based on the “filter” of applied tags.