Mastering Terraform Contains and Strcontains Functions

contains(): is the value in there, or are you debugging all weekend#

contains() checks whether a specific value sits inside a list or a set.

1
contains(list, value) => bool

Returns true if the list has the value. false if it doesn’t. That’s the whole API.

Basic? Yes. But once your infra logic starts leaning on user input, region capabilities, or feature flags, this little function turns into the bouncer at the door of your Terraform nightclub.

1
variable "region" {
2
  default = "uksouth"
3
}
4

5
variable "vm_size" {
6
  default = "Standard_DS2_v2"
7
}
8

9
data "azurerm_virtual_machine_sizes" "example" {
10
  location = var.region
11
}
12

13
output "is_supported" {
14
  value = contains(data.azurerm_virtual_machine_sizes.example.sizes, var.vm_size)
15
}

strcontains(): when you’re parsing strings like it’s bash again#

Now strcontains(). This is Terraform answering one question: does this string have that other string inside it?

1
strcontains(string, substr) => bool

Reach for it when there’s no list, just a single string. An AZ name, a tag, a label. You want to match a pattern in it.

1
strcontains("us-east-1b-optimal", "optimal") // returns true

1
locals {
2
  is_optimal_zone = strcontains(var.availability_zone, "optimal")
3
}

Gotchas that bit me, so they don’t bite you#

• contains() wants the exact value. Case-sensitive. No fuzzy matching.
• strcontains() doesn’t do regex or wildcards. Pure substring, nothing more.
• Maps don’t behave with contains() the way you’d hope. Lists and sets only. Try contains({ key = "val" }, "key") and you’ll be disappointed.
• Nulls can wreck your results. Validate your variables, or wrap them in coalesce() when it matters.

Bonus: validate with style#

Want to enforce rules on inputs? Drop these functions into validation blocks.

1
variable "az" {
2
  type    = string
3
  default = "us-east-1b-optimal"
4

5
  validation {
6
    condition     = strcontains(var.az, "optimal")
7
    error_message = "Only 'optimal' AZs are allowed for this deployment."
8
  }
9
}

Now Terraform fails early, with a message a human can read, instead of crashing halfway through the plan.

TL;DR#

• Reach for contains() with lists or sets.
• Reach for strcontains() for substring checks on a single string.
• Pair them with validation, count, or for_each for logic that stays clean and safe inside your modules.
• Don’t guess. Test.