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Terraform

Terraform: Check if String Contains Substring in Interpolation

By Filip on 10/07/2024

Learn how to efficiently check for substrings within strings using Terraform interpolation to enhance your infrastructure-as-code capabilities.

Terraform: Check if String Contains Substring in Interpolation

Table of Contents

Introduction

In Terraform, determining if a string contains a specific substring involves using the regexall function and a conditional check.

Step-by-Step Guide

To determine if a string contains a specific substring in Terraform, you can use the regexall function along with a conditional check.

  1. Utilize the regexall function. This function searches a given string for a matching pattern. It returns a list of matches found. For instance:

    regexall("substring", "This string contains substring")

    This would return a list containing one match: ["substring"].

  2. Verify the length of the resulting list. If the list's length is greater than zero, it indicates that the substring was found within the string. You can achieve this using the length function:

    length(regexall("substring", "This string contains substring")) > 0

    This expression evaluates to true because the regexall function finds one match.

  3. Incorporate this logic into conditional statements. You can use this approach within Terraform's conditional logic to control resource creation or variable assignments based on the presence or absence of a substring. For example:

    resource "example_resource" "example" {
  count = length(regexall("substring", var.input_string)) > 0 ? 1 : 0
  // ... other resource properties
}

    In this scenario, the example_resource will only be created if the variable var.input_string contains the substring "substring".

This method effectively leverages Terraform's built-in functions to determine if a string contains a specific substring, allowing for dynamic and conditional behavior in your infrastructure code.

Code Example

The code defines a Terraform configuration that checks if a given input string contains specific substrings. It uses the regexall function to find all occurrences of the substrings and the length function to count the matches. Based on the match count, it conditionally creates null_resource blocks, which execute local commands to print messages indicating the presence or absence of the substrings in the input string.

# Define a variable to hold the input string
variable "input_string" {
  default = "This string contains the word example."
}

# Check if the input string contains the substring "example"
resource "null_resource" "example_check" {
  # The resource will only be created if the condition is true
  count = length(regexall("example", var.input_string)) > 0

  provisioner "local-exec" {
    command = "echo 'The input string contains the word \"example\"'"
  }
}

# Check if the input string contains the substring "test"
resource "null_resource" "test_check" {
  # The resource will only be created if the condition is true
  count = length(regexall("test", var.input_string)) > 0

  provisioner "local-exec" {
    command = "echo 'The input string contains the word \"test\"'"
  }
}

Explanation:

  1. Variable Definition: We define a variable input_string to store the string we want to search within.
  2. regexall Function: We use the regexall function to search for the desired substring ("example" and "test" in this case) within the input_string.
  3. length Function: The length function is used to determine the number of matches found by regexall.
  4. Conditional Logic: We use the count meta-argument within the null_resource block to conditionally create resources.
    • If the length of the regexall result is greater than 0 (meaning a match was found), the count will be 1, and the resource will be created.
    • If no match is found, the count will be 0, and the resource will not be created.
  5. Output: Each null_resource has a local-exec provisioner that will print a message if the corresponding substring is found in the input_string.

This example demonstrates how to use regexall and conditional logic to control resource creation based on the presence or absence of specific substrings within a string. You can adapt this pattern to various scenarios in your Terraform code.

Additional Notes

Beyond regexall:

  • contains Function: For simple substring checks without regex, use the contains function. It's more efficient than regexall for basic scenarios.
  • strcontains Function: Similar to contains, but specifically for strings. Useful for clarity when dealing with mixed data types.

Regex Considerations:

  • Case Sensitivity: regexall is case-sensitive by default. Use flags within the regex pattern for case-insensitive matching.
  • Special Characters: Escape special characters in your substring if they have regex meaning (e.g., . or *).
  • Performance: Complex regex patterns can impact performance, especially with large strings. Consider simpler alternatives if possible.

Alternative Approaches:

  • Terraform Modules: For reusable substring checks, create a module that encapsulates the logic and accepts the string and substring as inputs.
  • External Data Sources: If your substring logic depends on external factors, use data sources to fetch the necessary information.

Best Practices:

  • Clear Variable Names: Use descriptive variable names to indicate the purpose of the string and substring being checked.
  • Comments: Add comments to explain the logic and purpose of the substring check, especially for complex regex patterns.
  • Testing: Thoroughly test your code with different input strings to ensure the substring check behaves as expected.

Summary

This table summarizes how to determine if a string contains a specific substring in Terraform:

Step Description Terraform Code Example
1. Use regexall Search a string for a matching pattern and return a list of matches. regexall("substring", "This string contains substring")
2. Check the List Length Determine if the substring was found by checking if the list length is greater than zero. length(regexall("substring", "This string contains substring")) > 0
3. Apply in Conditional Logic Use the result within conditional statements to control resource creation or variable assignments. terraform<br>resource "example_resource" "example" {<br> count = length(regexall("substring", var.input_string)) > 0 ? 1 : 0<br> // ... other resource properties<br>}

Key Points:

  • regexall returns a list of all matches found.
  • length function checks the length of the resulting list.
  • Conditional logic allows for dynamic behavior based on substring presence.

Conclusion

By combining the regexall function with conditional checks, Terraform enables you to create dynamic and adaptable infrastructure deployments. Whether using simple comparisons or more complex regular expressions, this approach offers a robust method for controlling resource provisioning and variable assignments based on the presence or absence of specific substrings within strings. Remember to leverage Terraform's built-in functions and best practices for code clarity and maintainability.

References

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