The hardest thing about writing a parser is cognitively accepting what is going to be considered valid input. You can make the best parser that is fast and well specified but invariably someone will (ab)use it in an unexpected way.
Famous examples: despite so many initial good intentions, html tags don’t need to be closed, JSON numbers are too often encoded as strings, YAML can look like what most people expect or it can look progressively more like JSON… and on and on.
Famous examples: despite so many initial good intentions, html tags don’t need to be closed, JSON numbers are too often encoded as strings, YAML can look like what most people expect or it can look progressively more like JSON… and on and on.