code: function getUrl (value) {
    const reg = /url\((\s*)(['"]?)(.+?)\2(\s*)\)/g
    const match = reg.exec(value)
    const url = match[URL_POSITION]
    return url
  }
docs: Extract the contents of a css url
  
  @param  {string} value raw css
  @return {string}       the contents of the url