Flex lexical analyser

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flex (fast lexical analyzer generator) is a free software alternative to Lex. It is frequently used with the free Bison parser generator. Flex was originally written in C by Vern Paxson around 1987.

The description for flex as given by the flex manual:

"flex is a tool for generating scanners: programs which recognize 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..."

A similar lexical scanner for C++ is flex++.

Flex is a non-GNU project, but the GNU project developed the manual for Flex.