Specify validation checks for CommaDelimitedList parameter with AWS CloudFormation
AWS CloudFormation now supports AllowedValues and AllowedPattern properties for CommaDelimitedList parameter type. You can use the CommaDelimitedList parameter type to specify multiple string values in a single parameter. For example, you can create three different subnets with their own VPC CIDR blocks, and use CommaDelimitedList to specify three different CIDR blocks. With these language enhancements, you can add validation checks to your CommaDelimitedList, and reduce input errors to your CommaDelimitedList while updating your stacks and/or stack sets.