How to Check if String Contains a Substring in Terraform Interpolation?

TLDR

To check if a string contains a substring in Terraform, use the can function with regex or the contains function for lists. These methods allow you to perform substring checks dynamically.

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Terraform provides several ways to check if a string contains a substring, depending on your use case. This guide explains how to achieve this using practical examples.

Step 1: Use the regex Function

The regex function allows you to match substrings using regular expressions.

Example

variable "example_string" {
  default = "hello world"
}

locals {
  contains_hello = can(regex("hello", var.example_string))
}

output "result" {
  value = local.contains_hello
}

Explanation

• regex("hello", var.example_string): Checks if the string contains the substring hello.
• can: Returns true if the regex matches, otherwise false.
• local.contains_hello: Stores the result of the check.

Step 2: Use the contains Function for Lists

If you are working with a list of strings, use the contains function.

Example

variable "example_list" {
  default = ["hello", "world"]
}

locals {
  contains_hello = contains(var.example_list, "hello")
}

output "result" {
  value = local.contains_hello
}

Explanation

• contains(var.example_list, "hello"): Checks if the list contains the string hello.
• local.contains_hello: Stores the result of the check.

Step 3: Combine with Conditional Logic

Use the result of the substring check in conditional logic.

Example

variable "example_string" {
  default = "hello world"
}

locals {
  message = can(regex("hello", var.example_string)) ? "Substring found" : "Substring not found"
}

output "result" {
  value = local.message
}

Explanation

• can(regex("hello", var.example_string)): Checks for the substring.
• ? "Substring found" : "Substring not found": Returns a message based on the result.

Best Practices

• Use Descriptive Variable Names: Clearly indicate the purpose of each variable.
• Validate Inputs: Ensure strings are in the expected format before performing checks.
• Document Logic: Provide clear comments for complex expressions.

By following these steps, you can effectively check if a string contains a substring in Terraform, enabling dynamic and flexible configurations.