Flex (The Fast Lexical Analyzer)
Flex is a fast lexical analyser generator. It is a tool for generating programs that perform pattern-matching on text. Flex is a non-GNU free implementation of the well known Lex program.The GNU Project did develop a manual for Flex. This documentation
can be found at
http://www.gnu.org/software/flex/manual/.
(This GNU documentation is for an old versions of Flex)