Package flex: Information

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 reentrant, 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.