(flex.info)Introduction

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3 Introduction
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'flex' is a tool for generating "scanners".  A scanner is a program
which recognizes lexical patterns in text.  The 'flex' program reads the
given input files, or its standard input if no file names are given, 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' by default, which
defines a routine 'yylex()'.  This file can be compiled and linked with
the flex runtime 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.