Lex/Flex tool overview in Compiler Design - Mini Project: Build & Apply

Lex/Flex Tool Overview

📖 Scenario: You are learning about how computers read and understand text. Lex and Flex are tools that help break text into pieces called tokens. This is useful when making programs that read languages or commands.

🎯 Goal: Build a simple Lex/Flex file that recognizes a few words and numbers. This will show how Lex/Flex works to identify parts of text.

📋 What You'll Learn

Create a Lex/Flex file with basic token definitions

Add rules to recognize keywords and numbers

Include a section to print recognized tokens

Complete the Lex/Flex file structure correctly

💡 Why This Matters

🌍 Real World

Lex/Flex tools are used to create programs that read and understand text, such as compilers and interpreters.

💼 Career

Understanding Lex/Flex helps in jobs related to programming language development, software tools, and text processing.

1

Create the Lex/Flex file header

Start by writing the first section of the Lex/Flex file. Create a section enclosed in %{ and %} where you include the stdio.h header. This section sets up the environment for your Lex/Flex program.

Compiler Design

%{
// Include standard input-output header
// Your code here
%}

Need a hint?

Use %{ and %} to start and end the header section. Inside, write #include <stdio.h>.

2

Add definitions for tokens

Add a definitions section after the header. Define two tokens: NUMBER as a pattern matching one or more digits ([0-9]+), and KEYWORD as the word if. Write these definitions using %option or simple pattern names.

Compiler Design

%{
#include 
%}

// Define tokens for NUMBER and KEYWORD
// Your code here

Need a hint?

Write NUMBER [0-9]+ and KEYWORD if after the header section.

3

Write rules to recognize tokens

Create the rules section by writing patterns and actions. For the if keyword, write a rule that matches if and prints Keyword: if. For numbers, write a rule that matches digits and prints Number: followed by the matched text. Use printf and yytext to print matched text.

Compiler Design

%{
#include 
%}

NUMBER [0-9]+
KEYWORD if

%%
// Add rules to print recognized tokens
// Your code here
%%

Need a hint?

Use %% to start and end the rules section. Write rules for if and {NUMBER} patterns with printf.

4

Add the main function to run the lexer

Add the final section with a main function. Inside main, call yylex() to start the lexer. This completes the Lex/Flex file so it can be compiled and run.

Compiler Design

%{
#include 
%}

NUMBER [0-9]+
KEYWORD if

%%
if      { printf("Keyword: if\n"); }
{NUMBER} { printf("Number: %s\n", yytext); }
%%

// Add main function to run lexer
// Your code here

Need a hint?

Write int main() with yylex(); inside to run the lexer.