There is nothing in the API documentation to suggest that
DescribeSubnets accepts a regular expression in filter names: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeSubnets.html
If it works in the CLI, that's likely something the CLI is doing on top of what the SDK offers. The Go SDK is like any other AWS SDK; it exposes the AWS API in a language-specific way. The AWS CLI adds convenience features on top of the API to make it more useful on the command line, but that doesn't mean those features are exposed by the API or any published SDK.