code: function format(identifier) {
    if(typeof identifier === 'string') {
      var hexIdentifier = identifier.replace(/[^A-Fa-f0-9]/g, '');
      return hexIdentifier.toLowerCase();
    }
    return null;
  }
docs: Convert the given identifier to a valid identifier string, if possible.
  @param {String} identifier The given identifier.
  @return {String} Valid identifier string, or null if invalid input.