Requirements are still a bit vague but I would probably start with a simple python script to process the text file and output the result. That can already be us

Requirements are still a bit vague but I would probably start with a simple python script to process the text file and output the result. That can already be used on computers on the command line. Then you could wrap it up in a website using django or another small web framework with an html form to upload the text file, process it using the same script and output the result as html. Then it becomes available on all devices with a web browser.