Using regular expressions in CloudFormation templates

You can use regular expressions (commonly known as regexes) in a number of places within your CloudFormation templates, such as for the AllowedPattern property when creating a template parameter.

All regular expressions in CloudFormation conform to the Java regex syntax. For a comprehensive description of the Java regex syntax and its constructs, see java.util.regex.Pattern.

If you write your CloudFormation template in JSON syntax, you must escape any backslash characters (\) in your regular expression by adding an additional backslash. This is because JSON interprets backslashes as escape characters, and you need to escape them to ensure they're treated as literal backslashes in the regular expression.

For example, if you include a \d in your regular expression to match a digit character, you will need to write it as \\d in your JSON template.

In the following example, the AllowedPattern property specifies a regular expression that matches four consecutive digit characters (\d{4}). However, since the regular expression is defined in a JSON template, the backslash character needs to be escaped with an additional backslash (\\d).

{
  "Parameters": {
    "MyParameter": {
      "Type": "String",
      "AllowedPattern": "\\d{4}"
    }
  }
}

If you write your CloudFormation template in YAML syntax, you must surround the regular expression with single quotation marks (''). No additional escaping is required.

Parameters:
  MyParameter:
    Type: String
    AllowedPattern: '\d{4}'