qtFlexC++ is a tool for generating scanners: programs which recognized lexical patterns in text. qtFlexC++ reads the given input files for a description of a scanner to generate. The description is in the form of pairs of regular expressions and C++ code, called rules. qtFlexC++ generates as output a C++ source and include file, lexyy.cpp and lexyy.h, which defines a class `lexyy'.
This file is compiled with your code to produce an executable. When the executable is run, it analyzes its input for occurrences of the regular expressions. Whenever it finds one, it executes the corresponding C++ code.