lib/classes/Variables.js/Variables/constructor
class Variables:  constructor(serverless) {
    this.serverless = serverless;
    this.service = this.serverless.service;
    this.tracker = new PromiseTracker();

    this.deep = [];
    this.deepRefSyntax = RegExp(/(\${)?deep:\d+(\.[^}]+)*()}?/);
    this.overwriteSyntax = RegExp(/\s*(?:,\s*)+/g);
    this.fileRefSyntax = RegExp(/^file\((~?[a-zA-Z0-9._\-/]+?)\)/g);
    this.envRefSyntax = RegExp(/^env:/g);
    this.optRefSyntax = RegExp(/^opt:/g);
    this.selfRefSyntax = RegExp(/^self:/g);
    this.stringRefSyntax = RegExp(/(?:('|").*?\1)/g);
    this.cfRefSyntax = RegExp(/^(?:\${)?cf:/g);
    this.s3RefSyntax = RegExp(/^(?:\${)?s3:(.+?)\/(.+)$/);
    this.ssmRefSyntax = RegExp(/^(?:\${)?ssm:([a-zA-Z0-9_.\-/]+)[~]?(true|false)?/);
  }