Using Terraform String ‘replace’ Function For RegEx String R…

HashiCorp Terraform offers a lot of powerful functions that can be used for string manipulation within your Infrastructure as Code (IaC) projects. Among these functions is the versatile replace function, which enables users to perform both simple string replacements as well as regular expressions (RegEx) based string replacement. In this article, we’ll look deeper into how to leverage Terraform’s replace function for RegEx based string replacement, along with some tips and common use cases.

Let’s dig in!

Look at a Simple String Replace in Terraform

Before diving into RegEx replacements, let’s first understand how the replace function works for simple string replacements.

The signature for the replace function is straightforward:

replace(string, substring, replacement)

• string is the source input string.
• substring is the string that represents the value to be replaced.
• replacement is the replacement string.

Here’s a simple example of performing a simple string replacement using the Terrafrom replace function:

local 
  value = replace(" "/http[s

output "simple-replace" 
  value = replace(" "/http[s
# Outputs: "Hello, Terraform!"

In this example, the output will be “Hello, Terraform!” as “world” is replaced with “Terraform”.

Using RegEx to Replace String Values in Terraform

While simple string replacements are effective for static substitutions, utilizing regular expressions (RegEx) introduces a level of sophistication and flexibility in string manipulation within Terraform configurations. RegEx allows you to define patterns that match specific sequences of characters, enabling dynamic replacements based on those patterns.

One of the key advantages of using RegEx with Terraform’s replace function is its ability to handle complex scenarios where the string to be replaced follows a certain pattern or varies in structure.

Direct RegEx String Replacement

With the Terraform replace function, if the substring value is wrapped in forward slashed, it tells Terraform to treat it as a regular expression (RegEx), using the same syntax that is supported by the Terraform regex function.

local ]?:\\/\\//", "")


output "replaced_string" 
  value = replace(" "/http[s
# Outputs: "Th* q**ck br*wn f*x j*mps *v*r th* l*zy d*g"

In this example, all vowels (a, e, i, o, u) in the input string are replaced with “*”. This example will output "Th* q*ck brwn fx jmps vr th* lzy dg" after the replace function executes.

Dynamic RegEx String Replacement with Captured Strings

If the regular expression (RegEx) used with the Terraform replace function, the replacement passed to the function can incorporate captured strings from the input string by using a $n sequence; where n is the index or name of a captured group.

Here’s an example of incorporating a captured string in the replacement with a RegEx based string replace:

output "regex-replace-captured-strings" {
  value = replace("John Doe (johndoe@example.com)", "/([a-zA-Z]+) ([a-zA-Z]+) \\(([^)]+)\\)/", "$2, $1 - Email: $3")
}
# Outputs: "Doe, John - Email: johndoe@example.com"

In this example, the input string represents a name followed by an email address enclosed in parentheses. The regular expression "/([a-zA-Z]+) ([a-zA-Z]+) \\(([^)]+)\\)/" is used to capture the first name, last name, and email address as separate groups.

• $1 refers to the first captured group, which is the first name.
• $2 refers to the second captured group, which is the last name.
• $3 refers to the third captured group, which is the email address.

In the replacement string "$2, $1 - Email: $3", these captured groups are incorporated into the final formatted string. The output of this will be "Doe, John - Email: johndoe@example.com".

This capability allows for dynamic replacements where the replacement string can be customized based on the content of the input string, providing greater flexibility in string manipulation within Terraform configurations.

Useful RegEx Patterns to use with Terraform String replace Function

Here are several useful RegEx patterns to use with Terraform’s replace function:

Hopefully these examples will either help you directly with the regular expression replacement you need, or at least give you some tips that can help you build the regular expressions you need.

Replace a Number

output "regex-replace-number" {
    value = "${replace("hello world 123", "/[0-9]/", "X")}"
}
# Output: "hello world XXX"

Replace Non-Alphanumeric Characters

output "regex-replace-non-alphanumeric" {
  value = replace("Hello! This is a test string #123$%^", "/[^a-zA-Z0-9]/", "")
}
# Outputs: "HelloThisisateststring123"

Isolate Domain Name from Email Address

output "regex-replace-email-domain" {
  value = replace("chris@build5nines.com", "/^[^@]+@([^@]+)$/", "$1")
}
# Outputs: "buildn5nines.com"

Remove Whitespace

output "regex-replace-remove-whitespace" {
  value = replace("This is a test", "/[\\s]+/", "")
}
# Outputs: "Thisisatest"

Remove HTTP(S) URL Protocol from Domain Name

output "regex-replace-url-protocol" {
  value = replace(" "/http[s|]?:\\/\\//", "")
}
# Outputs: "build5nines.com"

Remove HTML Tags

output "regex-replace-remove-html-tags" {
  value = replace("
Hello, world!
", "/<[^>]+>/", "")
}
# Outputs: "Hello, world!"

Normalize Phone Number

output "regex-replace-normalize-phone-number" {
  value = replace("+1 (555) 123-4567", "/[^\\d]/", "")
}
# Outputs: "15551234567"

Original Article Source: Using Terraform String ‘replace’ Function for RegEx String Replacement by Chris Pietschmann (If you’re reading this somewhere other than Build5Nines.com, it was republished without permission.)