Introduction to the elm/parser package

Do you need to deal with some kind of structured data which is not in JSON format and too complex for regular expressions? Or perhaps you have to validate some kind of complex user input? Or maybe you need to report to users what exactly is wrong with the data?
One of the packages in the Elm core library which can help you with such tasks is elm/parser.
First of all, if you're not familiar with parsing, what is it? A parser allows us to take a string as input and to convert it into Elm values a...