## Custom Validation Rules

In addition to Type Constraints, you can specify arbitrary custom validation
rules for a particular variable using one or more `validation` block nested
within the corresponding `variable` block:

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

  validation {
    condition     = length(var.image_id) > 4 && substr(var.image_id, 0, 4) == "ami-"
    error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
  }
}
```

The `condition` argument is an expression that must use the value of the
variable to return `true` if the value is valid or `false` if it is invalid.
The expression can refer only to the variable that the condition applies to,
and _must not_ produce errors.

If the failure of an expression is the basis of the validation decision, use
[the `can` function](/docs/templates/hcl_templates/functions/conversion/can) to detect such errors. For example:

```hcl
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-\"."
  }
}
```

If `condition` evaluates to `false`, an error message including the sentences
given in `error_message` will be produced. The error message string should be
at least one full sentence explaining the constraint that failed, using a
sentence structure similar to the above examples.

Validation also works with more complex cases:

```hcl
variable "image_metadata" {

  default = {
    key: "value",
    something: {
      foo: "bar",
    }
  }

  validation {
    condition     = length(var.image_metadata.key) > 4
    error_message = "The image_metadata.key field must be more than 4 runes."
  }

  validation {
    condition     = can(var.image_metadata.something.foo)
    error_message = "The image_metadata.something.foo field must exist."
  }

  validation {
    condition     = substr(var.image_metadata.something.foo, 0, 3) == "bar"
    error_message = "The image_metadata.something.foo field must start with \"bar\"."
  }

}
```
