CloudFormation - Search & Replace Custom Resource

example.yml

---
AWSTemplateFormatVersion: 2010-09-09
Description: Testing something

Resources:
  R1:
    Type: Custom::StringReplacement
    Properties:
      ServiceToken: !ImportValue cfn-helpers:Replace
      Find: '-test'
      Replace: '-prod'
      Subject: example-stack-name-test
  
  R2:
    Type: Custom::StringReplacement
    Properties:
      ServiceToken: !ImportValue cfn-helpers:Replace
      Find: '-[^-]+$'
      Limit: 1
      Regex: True
      Replace: '-prod'
      Subject: example-stack-name-test

Outputs:
  R1:
    # Results in 'example-stack-name-prod'
    Value: !GetAtt R1.Result
  R2:
    # Results in 'example-stack-name-prod'
    Value: !GetAtt R2.Result

# vim: ft=yaml.cloudformation sw=2 ts=2

template.yml

---
AWSTemplateFormatVersion: 2010-09-09
Description: |
  Provides a Lambda-backed Custom Resource for doing string replacement.
  
  Resource Parameters:
    Find (required, string): String or regular expression to find
    Insensitive (optional, boolean): Switch to indicate case sensitivity.
        Defaults to False & requires Regex=True
    Limit (optional, integer): Maximum number of matches to replace
    Regex (optional, boolean): If the `Find` parameter is a regular
        expression. Defaults to False
    Replace (required, string): Replacement string
    Subject (required, string): String to do replacement in

  Return Parameters:
    Result (string): Result of the string replacement

Resources:
  ReplaceFunctionLogGroup:
    Type: AWS::Logs::LogGroup
    Properties:
      LogGroupName: !Sub /aws/lambda/${ReplaceFunction}
      RetentionInDays: 1

  ReplaceFunctionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal: {Service: lambda.amazonaws.com}
            Action: sts:AssumeRole
      Path: /cfn-custom-resources/
      Policies:
        - PolicyName: logs
          PolicyDocument:
            Statement:
              - Effect: Allow
                Action:
                  - logs:CreateLogStream
                  - logs:PutLogEvents
                Resource: !Sub "arn:aws:logs:${AWS::Region}:${AWS::AccountId}:*"

  ReplaceFunction:
    Type: AWS::Lambda::Function
    Properties:
      Description: Executes string replacement
      Handler: index.lambda_handler
      MemorySize: 128
      Role: !GetAtt ReplaceFunctionRole.Arn
      Runtime: python3.6
      Timeout: 5
      Code:
        ZipFile: |
          import cfnresponse


          def lambda_handler(event, context):
              # Define the signal & data that will be returned
              return_status = cfnresponse.FAILED
              return_data = {}

              try:
                  # Fetch the invocation settings
                  props = event['ResourceProperties']
                  find = props['Find'] 
                  insensitive = props.get('Insensitive', '').lower() == 'true'
                  limit = int(props.get('Limit', 0))
                  regex = props.get('Regex', '').lower() == 'true'
                  replace = props['Replace']
                  subject = props['Subject']

                  # Sanity check
                  if insensitive and not regex:
                      raise NotImplementedError(
                            'Cannot do case-insensitive replacement without'
                            'setting Regex=True')

                  if regex:
                      # Regular expression replacement
                      import re
                      kwargs = {}
                      if limit:
                          kwargs.update({'count': limit})
                      if insensitive:
                          kwargs.update({'flags': re.I})

                      result = re.sub(find, replace, subject, **kwargs)
                  else:
                      # Simple string replacement
                      result = subject.replace(find, replace)

                  # Setup return data & signal
                  return_data.update({
                      'Result': result,
                  })
                  return_status = cfnresponse.SUCCESS

              except Exception as e:
                  # Some error
                  import traceback
                  traceback.print_exc()
                  return_data.update({
                      'Exception': str(e),
                  })

              # Send the CFN response
              cfnresponse.send(
                  event,
                  context,
                  return_status,
                  return_data)

Outputs:
  ReplaceFunctionArn:
    Description: ARN of the replacement Lambda function
    Value: !GetAtt ReplaceFunction.Arn
    Export:
      Name: cfn-helpers:Replace

# vim: ft=yaml.cloudformation sw=2 ts=2