What is Lexer: Definition, How It Works, and Examples

A lexer is a program component that breaks input text into meaningful pieces called tokens. It reads raw text and groups characters into words, numbers, or symbols to help a compiler or interpreter understand the code.

How It Works

A lexer works like a scanner that reads text from left to right and groups characters into chunks called tokens. Imagine reading a sentence and separating it into words and punctuation marks. The lexer does this automatically for programming languages, turning raw code into pieces like keywords, numbers, or operators.

Each token has a type and value, such as a number token with the value "123" or a keyword token like "if". This makes it easier for the next step, called parsing, to understand the structure and meaning of the code.

Example

This simple Python example shows a lexer that splits a string into tokens of words and numbers.

python

import re

def simple_lexer(text):
    token_specification = [
        ('NUMBER',   r'\d+'),    # Integer number
        ('WORD',     r'[A-Za-z]+'), # Words
        ('SKIP',     r'[ \t]+'),  # Skip spaces and tabs
        ('MISMATCH', r'.'),       # Any other character
    ]
    tok_regex = '|'.join(f'(?P<{name}>{pattern})' for name, pattern in token_specification)
    for mo in re.finditer(tok_regex, text):
        kind = mo.lastgroup
        value = mo.group()
        if kind == 'SKIP':
            continue
        elif kind == 'MISMATCH':
            raise RuntimeError(f'Unexpected character: {value}')
        else:
            print(f'{kind}: {value}')

# Example usage
simple_lexer('if x == 42')

Output

WORD: if
WORD: x
MISMATCH: =

When to Use

Lexers are used whenever you need to process and understand text with a specific structure, especially programming languages. They are the first step in compilers and interpreters to convert code into a form that machines can analyze.

Besides compilers, lexers are useful in text editors for syntax highlighting, data validation, and any tool that needs to break down input into meaningful parts.

Key Points

• A lexer splits raw text into tokens like words, numbers, or symbols.
• It simplifies the next step of understanding code called parsing.
• Lexers are essential in compilers, interpreters, and text processing tools.
• Tokens have types and values that describe their role in the text.

Key Takeaways

A lexer breaks input text into tokens to simplify code analysis.

Tokens represent meaningful elements like keywords, numbers, or symbols.

Lexers are the first step in compilers and interpreters.

They are also used in tools like syntax highlighters and validators.