How to Build Your Own Compiler from Scratch

9 February 2026

Have you ever wondered how your computer actually understands the programs you write? You type some code, press a button, and—boom—it runs! Behind the scenes, there's a powerful program called a compiler that translates your human-friendly code into machine-friendly instructions.

Building a compiler sounds like something only programming wizards do, right? Wrong! While it’s definitely a challenging project, creating your own compiler from scratch is one of the most rewarding experiences you can have as a developer. It deepens your understanding of programming languages, computer architecture, and even how your favorite languages like C++ or Python work under the hood.

If you're ready, let’s roll up our sleeves and demystify the magic behind compilers! 🚀

What is a Compiler, Really?

Before we dive in, let's clarify what a compiler actually does. In simple terms, a compiler takes code written in a high-level programming language (like C, Java, or Python) and converts it into machine code that the CPU can execute.

It's like having a translator who takes your English instructions and turns them into machine language spoken in ones and zeros.

A compiler works in multiple stages, each responsible for a different part of the translation process. These stages include:

1. Lexical Analysis - Breaking the source code into tokens (smallest meaningful units like keywords, variables, operators).
2. Syntax Analysis (Parsing) - Ensuring that the structure of the code follows the grammar rules of the language.
3. Semantic Analysis - Checking for logical correctness, like making sure variables are used properly.
4. Intermediate Code Generation - Creating an intermediate representation (IR) to make further optimizations easier.
5. Optimization - Improving the IR to make the program run faster and use fewer resources.
6. Code Generation - Converting the optimized IR into actual machine code.

Now that we understand what a compiler does, let's look at the roadmap to building our own!

Step 1: Choose Your Source and Target Language

Before you start writing hundreds of lines of code, you need to decide:

- What language will your compiler translate? (e.g., a simple language like TinyLang, or an existing one like Python)
- What will it output? (e.g., assembly language, bytecode, or directly to machine code)

For beginners, it’s best to create a compiler for a simple toy language and output assembly code. This helps you focus on learning without getting lost in complexity.

Step 2: Tokenizing the Input (Lexical Analysis)

The first step in any compiler is lexical analysis, where we break the source code into tokens. Think of tokens as puzzle pieces; once we have them, we can figure out how they fit together.

For example, given this simple code snippet:

c
int x = 5;

A lexer (or lexical analyzer) would break it down into tokens like:

- `int` → keyword
- `x` → identifier (variable name)
- `=` → assignment operator
- `5` → number
- `;` → end of statement

You can implement a lexer using simple regular expressions or even a finite state machine. Most lexers scan through the input character by character, grouping them into meaningful units.

Here's a tiny example in Python:

python
import re  
TOKEN_REGEX = [
    (r'\bint\b', 'KEYWORD'),
    (r'[a-zA-Z_][a-zA-Z0-9_]*', 'IDENTIFIER'),  
    (r'\d+', 'NUMBER'),  
    (r'=', 'ASSIGNMENT'),  
    (r';', 'SEMICOLON'),
    (r'\s+', None)  


Ignore whitespace
]
def lexer(code):
    tokens = []
    while code:
        for pattern, tag in TOKEN_REGEX:
            match = re.match(pattern, code)
            if match:
                if tag:
                    tokens.append((tag, match.group(0)))
                code = code[len(match.group(0)):]
                break
        else:
            raise SyntaxError("Unexpected character: " + code[0])
    return tokens
print(lexer("int x = 5;"))

This will output:

bash
[('KEYWORD', 'int'), ('IDENTIFIER', 'x'), ('ASSIGNMENT', '='), ('NUMBER', '5'), ('SEMICOLON', ';')]

Pretty cool, right? Let’s keep going!

Step 3: Parsing (Syntax Analysis)

Now that we have tokens, we need to ensure they form valid code. This is the job of the parser, which builds a structure called an Abstract Syntax Tree (AST).

An AST is like a family tree for code. For example, the statement `int x = 5;` would be structured like this:

Assignment
├── Type: int
├── Variable: x
└── Value: 5

Parsers follow formal grammar rules to organize tokens correctly. A recursive descent parser is a simple way to implement this.

Here’s a quick sketch of how a parser might convert our tokens into an AST:

python
class ASTNode:
    def __init__(self, type, value=None):
        self.type = type
        self.value = value
        self.children = []
def parse(tokens):
    if tokens[0][0] == "KEYWORD" and tokens[0][1] == "int":
        var_name = tokens[1][1]
        value = tokens[3][1]
        return ASTNode("Assignment", {"var_name": var_name, "value": value})
tokens = [('KEYWORD', 'int'), ('IDENTIFIER', 'x'), ('ASSIGNMENT', '='), ('NUMBER', '5'), ('SEMICOLON', ';')]
ast = parse(tokens)
print(ast.type, ast.value)

This organizes our tokens into a neat structure, making execution easier later! 🎯

Step 4: Generating Intermediate Code

At this stage, we take our AST and generate intermediate code—a lower-level representation of our program that’s easier to optimize and translate into machine code.

A common choice is Three-Address Code (TAC), which looks like:

t1 = 5
x = t1

This makes optimization simpler because we have clear instructions.

Step 5: Optimization

Compilers optimize the intermediate code to make execution faster and more efficient. Some basic optimizations include:

✅ Constant Folding → Replacing expressions like `2 + 3` with `5` at compile time.
✅ Dead Code Elimination → Removing unused variables or unreachable code.
✅ Loop Unrolling → Optimizing loops to avoid unnecessary jumps.

Simple optimizations can dramatically boost performance! 🚀

Step 6: Code Generation

Finally, we convert the optimized intermediate code into actual machine code or assembly language.

If you’re compiling to x86 assembly, your final output might look like:

assembly
mov eax, 5
mov x, eax

Boom—your compiler just turned human-readable code into CPU instructions! 🎉

Step 7: Running Your Compiled Code

At this point, you can save your generated assembly code to a file, assemble it using `nasm`, and execute it on your system.

For example, if you output an assembly file `output.asm`, you can compile it using:

bash
nasm -f elf64 output.asm -o output.o
gcc output.o -o output
./output

You’ve built a compiler from scratch—congratulations! 🎊

Final Thoughts

Building a compiler might seem like climbing Mount Everest, but taking it one step at a time makes it manageable. You now have a foundation to expand upon—whether it's adding more features, optimizing performance, or even compiling real-world languages.

Trust me, once you understand compilers, programming feels like you’ve unlocked developer superpowers.