Since version 0.13, terraform has support for custom validation rules for input variables.

The example in the documentation shows how to test a single value:

variable "image_id" {
  type        = string
  description = "The id of the machine image (AMI) to use for the server."

  validation {
    # regex(...) fails if it cannot find a match
    condition     = can(regex("^ami-", var.image_id))
    error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."

But, what to do if you want to validate a more complex object, such as list(string) (or other, more complicated types)? Terraform 0.14 introduced the alltrue function that makes this much easier and readable:

variable "aliases" {
  description = "List of any aliases (CNAMEs) for the website."
  type        = list(string)
  default     = []
  validation {
    condition = alltrue([
      for alias in var.aliases : can(regex("^[.0-9a-z-]+$", alias))
    error_message = "Aliases must be a valid DNS name."