1. 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. This product includes software developed by the University of California, Berkeley and its contributors. The upstream source code can be found at ftp://ftp.gnu.org/pub/non-gnu/flex/ From Debian 3.0r0 APT

2. The flex program generates scanners. Scanners are programs which can recognize lexical patterns in text. Flex takes pairs of regular expressions and C code as input and generates a C source file as output. The output file is compiled and linked with a library to produce an executable. The executable searches through its input for occurrences of the regular expressions. When a match is found, it executes the corresponding C code. Flex was designed to work with both Yacc and Bison, and is used by many programs as part of their build process. From Redhat 8.0 RPM

3. fast lexical analyzer generator From whatis