You can add validation rules to ensure that input values meet certain criteria.

variable "instance_type" {
  description = "Type of EC2 instance"
  type        = string
  default     = "t2.micro"
 
  validation {
    condition     = contains(["t2.micro", "t2.small", "t2.medium"], var.instance_type)
    error_message = "Instance type must be one of t2.micro, t2.small, or t2.medium."
  }
}