IP addresses can not be expressed in a defanged format

[CyberDaedalus00]

I was attempting to express some IPv4 addresses in a typical defanged format but received errors that it didn't match the regex for a IPv4 in CIDR format. When I looked at the regex expression, it appeared that it didn't support a common approach of encasing at least one '.' within '[' ']'. When I looked through the specification I also could not find any guidance as to how STIX 2.1 wants defanged IP addresses to be expressed.

What is the official syntax to expressing defanged IP addresses?