main.tf

## cat main.tf

variable "vpc_id" {
  type        = string
  description = "VPC ID to deploy to"

  validation {
    #condition     = can(regex("^vpc-", var.vpc_id))
    condition     = length(var.vpc_id) > 4 && substr(var.vpc_id, 0, 4) == "vpc-"
    error_message = "The vpc_id must be a valid VPC ID of the form 'vpc-'."
  }
}

####################### Pass an invalid value

./terraform plan -var vpc_id=foo

Error: Invalid value for variable

  on main.tf line 1:
   1: variable "vpc_id" {

The vpc_id must be a valid VPC ID of the form 'vpc-'.

This was checked by the validation rule at main.tf:6,3-13.

####################### Pass a valid value

./terraform plan -var vpc_id=vpc-200
No changes. Infrastructure is up-to-date.

# Other neat example - is something valid yaml?
condition = var.custom_vars != "" ? can(yamldecode(var.custom_vars)) : true