When you are working with Terraform, ensuring accurate and validated input is the right way to build a reliable infrastructure. Regular infrastructure, also known as Regex, is a powerful way to match, filter, and validate the strings that are within the Terraform configurations. Whether you are working to verify the input variables or filtering the outputs, Regex can streamline the Infrastructure as Code (IaC) workflows.
Terraform regex has multiple built-in functions, like regex, regexall, and regex-based validations offer enough flexibility to handle many common and advanced use cases.
In this guide, we will walk you through everything that you need to master regex in Terraform, from the basic syntax to the real-world examples.
Why Use Regex in Terraform?
Terraform configurations often need strict validation and filtering of the inputs to avoid deployment errors and maintain consistent infrastructure. Regular expressions (regex) is a compact and flexible tool to:
- Validate input variables, such as resource names, IP addresses, or other identifiers.
- Enforce naming rules and input conventions to align the organisational policies.
- Filter lists or outputs dynamically based on patterns.
- Exact meaningful information from other strings to automate workflows.
Using Terraform regex helps you catch the errors early in the deployment pipeline, which will make your infrastructure more strong and easy to manage.
Terraform Regex Function: Syntax & Use
Terraform offers two major regex functions:
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- regex(pattern, string) — Returns a list of captured groups from the first match or an error if no match is found.
- regexall(pattern, string) — Returns a list of all non-overlapping matches of the pattern in the string.
Terraform regexall Syntax Example:
regexall(pattern, string)
- pattern – The regular expression pattern you want to match.
- string – The input string to search within.
- Returns – A list of all non-overlapping matches found.
Example:
locals {
input = “env-prod, env-dev, env-stage”
matches = regexall(“env-[a-z]+”, local.input)
}
This example captures all substrings in local.input that start with env- followed by lowercase letters.
Terraform regex Syntax Example:
regex(pattern, string)
- pattern – The regular expression pattern to match.
- string – The input string to search.
- Returns – A list containing the first match’s captured groups. If there are no capturing groups, it returns the entire matched string. Throws an error if no match is found.
Example:
locals {
version_string = “v1.2.3”
match_groups = regex(“v([0-9]+)\\.([0-9]+)\\.([0-9]+)”, local.version_string)
}
The functions are pretty useful for tasks, such as pattern matching within strings, enabling complex validations or data extractions within the Terraform modules.
Related Article: How to Use grep Regex for Powerful Search in Linux
Terraform Validation Regex: Enforcing Input Constraints
Terraform’s variable blocks support validation arguments that allow you to enforce the input constraints using Terraform regex. This ensures that the users will provide valid values before any resource is provisioned.
Example:
variable “environment” {
type = string
validation {
condition = can(regex(“^prod|dev|stage$”, var.environment))
error_message = “Environment must be one of: prod, dev, or stage.”
}
}
The validation enforces that the environment can only be prod, dev, or stage. Using Terraform regex in validations helps reduce configuration errors and enforces compliance with your infrastructure standards.
Filtering with Regex in Terraform
Terraform regex supports filtering of lists using the regexall function combined with for expressions or filter functions to select the elements for matching a pattern.
Example filtering list items starting with “app-”:
locals {
names = [“app-1”, “db-1”, “app-2”, “cache”]
filtered_names = [for name in local.names : name if length(regexall(“^app-“, name)) > 0]
}
This filtered_names list will only contain “app-1” and “app-2”.
Filtering with regex is especially useful for dynamic resource selection or conditional logic within Terraform configurations.
Practical Terraform Regex Examples
To understand the power of Terraform regex, it is best to see them in action. Here a few real-world use cases where Terraform regex can help streamline configurations, enforce standard and improve the code maintainability
- Validating IP Address Format
variable “ip_address” {
type = string
validation {
condition = can(regex(“^([0-9]{1,3}\\.){3}[0-9]{1,3}$”, var.ip_address))
error_message = “The IP address must be in a valid IPv4 format, e.g., 192.168.0.1.”
}
}
This ensures that the user will enter an IP address with four octets separated by the dots.
- Extracting Resource Identifiers
locals {
arn = “arn:aws:ec2:us-west-1:123456789012:instance/i-0abcd1234efgh5678”
instance_id = regex(“instance\\/(i-[a-zA-Z0-9]+)”, local.arn)[0]
}
This extracts i-0abcd1234efgh5678 from the AWS ARN.
- Enforcing S3 Bucket Naming Rules
variable “bucket_name” {
type = string
validation {
condition = can(regex(“^[a-z0-9.-]{3,63}$”, var.bucket_name))
error_message = “Bucket names must be 3-63 characters long, using only lowercase letters, numbers, dots, or hyphens.”
}
}
This will prevent the users from using uppercase letters or special characters that would cause the deployments to fail.
Example: Using the Terraform regexall Function
The regexall function is useful when you want to match all occurrences of a pattern in a string—not just the first.
Example: Extracting All Email Addresses from a String
locals {
text = “Contact [email protected] or [email protected] for assistance.”
emails = regexall(“[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}”, local.text)
}
This technique will enable you to extract multiple values from the logs, metadata, or user defined variables.
Common Errors and Troubleshooting Tips For Terraform Regex
| Error | Cause | Troubleshooting Tip |
| No match found error | The regex pattern does not match the input string. | Double-check your regex pattern syntax; test it with online regex testers. |
| Terraform validation always fails | The regex pattern or condition logic is incorrect. | Use can() to safely check regex matches and avoid hard failures. |
| Escaping issues | Special characters like . or \ not escaped properly. | Escape special characters with \\ inside strings in Terraform. |
| Unexpected empty list from regexall | Pattern matches nothing in the input string. | Confirm the pattern and input string format; try simpler regex to debug. |
| Misusing regex vs regexall | Using regex when multiple matches are expected or vice versa. | Use regex for single match with capture groups, regexall for all matches. |
| Terraform throws syntax errors | Improper use of regex syntax inside HCL. | Verify regex pattern correctness and ensure it fits Terraform’s HCL syntax. |
Conclusion
Terraform regex or regular expressions are a strong tool to enhance the configurations by enabling precise input validation, dynamic filtering, and complex string matching. Understanding the regex functions like regex and regexall will improve the reliability of your infrastructure deployments but will also enforce consistency and reduce the manual errors.
Frequently Asked Questions
What’s the difference between regex and regexall in Terraform?
Regex returns the first match of a regular expression, while regexall returns all matches in a list. Both are useful for extracting or validating text patterns in input data.
How can I validate variable input using regex in Terraform?
You can use the validation block in a variable definition along with regex to ensure input follows a specific format, such as email addresses, tags, or naming conventions.
What are common regex-related errors in Terraform?
Typical issues include incorrect regex syntax, trying to match null values, or using regex when multiple matches are needed (use regexall instead).
