I don't know if anyone else has tried using flex to generate an input scanner but its surprisingly useful.'flex is a tool for generating scanners: programs which recognized lexical patterns in text. flex reads the given input files, or its standard input if no file names are given, for a description of a scanner to generate. The description is in the form of pairs of regular expressions and C code, called rules. flex generates as output a C source file, `lex.yy.c', which defines a routine `yylex()'. This file is compiled and linked with the `-lfl' library 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.'
so basically if youve got some input string into your application you want to break the input into easily defined actions flex is the tool to use.

http://www.gnu.org/software/....html#SEC4