GNU FLEX Programming Language Help For Students

Introduction to GNU FLEX

Flex is a tool for generating scanners. A scanner, sometimes called a tokenizer, is a program which recognizes lexical patterns in text. The flex program reads user-specified 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 a C source file named, "lex.yy.c", which defines the function yylex(). The file "lex.yy.c" can be compiled and linked to produce an executable. When the executable is run, it analyzes its input for occurrences of text matching the regular expressions for each rule. Whenever it finds a match, it executes the corresponding C code.

Example: Lexes our input without using Bison to parse it:

{`
%{
#include
using namespace std;
#define YY_DECL extern "C" int yylex()

%}

%%

[ \t\n]         ;

[0-9]+\.[0-9]+  { cout << "Found a floating-point number:" << yytext << endl; }

[0-9]+          { cout << "Found an integer:" << yytext << endl; }

[a-zA-Z0-9]+    { cout << "Found a string: " << yytext << endl; }

%%

main() {

// lex through the input:

yylex();

}
`}

Compiler Download

To get started, see Download and install GNU FLEX and follow the instructions.