Unfortunately, your browser doesn't support the features that are required to run atto. If you're on mobile, try using atto on a desktop computer. Otherwise, please use the latest version of Chrome or Firefox, which are known to work with atto.
If you're using Safari, then it's doubtful that Apple will ever implement the required features due to their constant reluctance to modernise their browser. If you work for Apple/are on the WebKit dev team, please implement lookbehind in regex so we can make atto work for everyone!