An Acronym for fast lexical analyzer generator.

Flex is used to generate C programs that perform pattern matching on text. It produces a single lex.yy.c file which defines a single yylex() function, which can compiled and linked with libfl to produce an executable that analyzes its input for occurrences of regular expres­sions. Whenever it finds one, it executes the correspond­ing C code.

It is usually used in conjunction with Bison. The Flex code then passes the tokens it finds to the yacc(1) style parser generated by Bison.

As a completely spurious example this is what the rules file could look like

%{

1. include <string.h>

%}

MATCHBOB "bob"|"BOB"

%%

/* pattern action */

{MATCHBOB} {printf("bob has been found!);}

%%

/*user supplied functions */

See also: flex(1) lex(1) bison(1) yacc(1)