A fast lexical analyzer generator. flex is a tool for generating scanners: programs which recognized lexical patterns in text. flex 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. 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. The behaviour of Flex has undergone a major change since version 2.5.4a; Flex scanners are now reenterant, you may have multiple scanners in the same program with differing sets of defaults, and they play nicer with modern C and C++ compilers. The Flip side is that Flex no longer conforms to the POSIX lex behaviour, and the scanners require conforming implementations when flex is used in ANSI C mode. The package flex-old provides the older behaviour. This product includes software developed by the University of California, Berkeley and its contributors. The upstream source code can be found at http://prdownloads.sourceforge.net/lex/flex-2.5.31.tar.bz2?download The project web page is at http://lex.sf.net/