code: function _makePlaceholder(selector, value) {
    return selector.replace(_replaceRegExp(value), (_, first, last) =>
      `${first}%${_.trim()}${last}`);
  }
docs: Private: turns a portion of a selector into a placeholder (adding a %)
  
  * `selector` {String} of the selector to replace
  * `value` {String} portion of the selector to convert into a placeholder
  
  Returns the transformed selector string {String}