Mastering Terraform Contains and Strcontains Functions

contains(): Check If It’s In There — Or Burn a Saturday Debugging#

The contains() function checks whether a specific value exists inside a list or a set.

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contains(list, value) => bool

It returns true if the list contains the value. false if it doesn’t. That’s it.

Sounds basic? Sure. But when your infra logic starts depending on user input, region capabilities, or feature flags, this little guy becomes the bouncer at the front of your Terraform nightclub.

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variable "region" {
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  default = "uksouth"
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}
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variable "vm_size" {
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  default = "Standard_DS2_v2"
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}
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data "azurerm_virtual_machine_sizes" "example" {
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  location = var.region
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}
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output "is_supported" {
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  value = contains(data.azurerm_virtual_machine_sizes.example.sizes, var.vm_size)
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}

strcontains(): When You’re Parsing Strings Like It’s Bash Again#

Now let’s talk about strcontains() — Terraform’s way of answering the question: “Does this string have that other string inside it?”

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strcontains(string, substr) => bool

Use it when you don’t have a list, just a single string — like an AZ name, tag, or label — and want to match patterns.

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

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

Gotchas (That Bit Me, So You Don’t Have To)#

• contains() cares about exact values. Case-sensitive. No fuzzy matches.
• strcontains() won’t match regex or wildcards. It’s pure substring.
• Maps don’t work with contains() the way you want. Only lists and sets. If you try contains({ key = "val" }, "key") — expect disappointment.
• Nulls can mess with results. Validate your variables, or wrap with coalesce() if needed.

Bonus: Validate with Style#

Want to enforce logic on inputs? Use validation blocks with these functions.

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variable "az" {
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  type    = string
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  default = "us-east-1b-optimal"
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  validation {
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    condition     = strcontains(var.az, "optimal")
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    error_message = "Only 'optimal' AZs are allowed for this deployment."
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  }
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}

Now Terraform fails early — with a message that makes sense — instead of crashing halfway through your infra plan.

TL;DR#

• Use contains() when working with lists or sets.
• Use strcontains() for substring checks in single strings.
• Combine them with validation, count, or for_each for clean, safe logic in your modules.
• Don’t guess. Test.