The old version of the fast lexical analyzer generator. flex is a tool for generating scanners: programs which recognize lexical patterns in text. This is the old 2.5.4a version, which is no longer being developed. You should normally choose flex, unless you have legacy lexer files that do not work with a modern flex. This product includes software developed by the University of California, Berkeley and its contributors. The upstream source code can be found at http://lex.sourceforge.net/