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📘 Lex (Flex) & Yacc (Bison) Setup and Basic Tutorial

1. Overview

This guide explains how to install and validate Lex/Yacc tooling on:

• Linux
• WSL (Windows Subsystem for Linux)
• macOS

Modern equivalents:

• Flex → lexical analyzer generator
• Bison → Yacc-compatible parser generator

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2. Installation

2.1 Ubuntu / WSL

sudo apt update
sudo apt install -y flex bison gcc make

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2.2 Fedora / RHEL

sudo dnf install -y flex bison gcc make

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2.3 macOS (Homebrew)

brew install flex bison

Add to PATH if needed:

# Apple Silicon
echo 'export PATH="/opt/homebrew/opt/bison/bin:$PATH"' >> ~/.zshrc
echo 'export PATH="/opt/homebrew/opt/flex/bin:$PATH"' >> ~/.zshrc
source ~/.zshrc

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3. Basic Verification

flex --version
bison --version
gcc --version

This only confirms installation—not correctness of the toolchain.

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✅ 4. Environment Validation (Critical Step)

This step ensures your full pipeline actually works:

• Flex → C code generation
• GCC → compilation
• Runtime → execution

4.1 Create Minimal Test

cat > test.l <<'EOF'
%{
#include <stdio.h>
%}

%%

hello   { printf("OK: FLEX WORKS\n"); }
.|\n    { }

%%

int main(void) {
    yylex();
    return 0;
}

int yywrap(void) { return 1; }
EOF

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4.2 Generate + Compile + Run

flex test.l
gcc lex.yy.c -o test
echo "hello" | ./test

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4.3 Expected Output

OK: FLEX WORKS

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4.4 Failure Diagnosis

Symptom | Likely Cause | Fix -- | -- | -- flex: command not found | Not installed / PATH issue | Reinstall or fix PATH gcc: command not found | Compiler missing | Install build-essential undefined reference to yywrap | Missing symbol | Add yywrap() or link -lfl No output | Rule not matching | Check regex or input

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7. Next Steps

• Integrate Flex with Bison
• Build a parser (calculator example)
• Generate AST and semantic checks

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8. Summary

The validation step (Section 4) is the most important addition:

✔ Confirms toolchain integrity
✔ Eliminates environment issues early
✔ Guarantees Flex → GCC → runtime pipeline works