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(flex.info)Description

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Description
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   `flex' is a tool for generating "scanners": programs which
recognized lexical patterns in text.  `flex' 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', 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.