title: Best Method to Manipulate a CfnParameter String in aws-cdk
body: <p>I'm trying in vain to do this. Here is the scenario:</p>
  
  <p>We are creating a CloudFormation Stack that will generate a CodePipeline, that will pull another stack definition from git and deploy it, using the <strong>CloudFormationCreateUpdateStackAction</strong></p>
  
  <p>The repo and branch etc. are provided as <strong>CfnParamaters</strong> and the subsequent stack name we would like to base off a concatenation of the repo name and branch name.
  However in some cases the repo might be named with an underscore or other special character that is invalid for a <strong>stackName</strong>.</p>
  
  <p>I've attempted to manipulate the parameter strings using Macro's but I didnt get anywhere near something useful and running a .replace() function on the <strong>repoStr.valueAsString</strong> property modifies the CDK's "Token" pointer, not the resulting paramter which is declared at runtime.</p>
  
  <p>Any ideas?</p>