Flex and Bison, the first is a lexical analyzer generator, and the the second a parser generator, two useful tools for making programs that reacts to a data input with a predefined structure and language, for example, we can make compilers, script interpreters and command line parsers.
This is not aimed to be a tutorial of Flex and Bison, if you want to learn how to use this programs I encourage reading their respective manuals, but a simple example on how to integrate these to Qt.