Lexical Analysis


In computer science, lexical analysis (also known as scanning or tokenization) is the first phase of compilation. It involves transforming a raw sequence of characters (the source code) into a meaningful sequence of tokens, according to the rules of a programming language's grammar. A program that performs lexical analysis is called a lexical analyzer, lexer, or scanner.

Key Tasks

  1. Reading Input: The lexical analyzer reads the source code character by character.
  2. Removing Noise: It ignores irrelevant elements like whitespace, comments, and preprocessor directives.
  3. Identifying Lexemes: The analyzer groups meaningful sequences of characters into units called lexemes. Examples include keywords ('if', 'while'), identifiers (variable names), operators ('+', '-'), or literals (numbers, strings).
  4. Converting to Tokens: Each lexeme is classified and converted into a token. A token is a structured representation consisting of:
    • Token Type: A category (e.g., "keyword," "identifier," "operator").
    • Token Value: The specific instance of the lexeme (e.g., the actual name of an identifier).
  5. Error Reporting: If the lexical analyzer encounters an invalid character sequence or an unrecognized pattern, it generates an error.


  • Regular Expressions: A powerful way to define the patterns of tokens. Many lexical analyzer generators (e.g., Lex, Flex) allow defining tokens using regular expressions.
  • Finite Automata: Finite-state machines are used to model and implement the logic of recognizing tokens. Deterministic finite automata (DFAs) are particularly efficient for lexical analysis.


Consider a code snippet:

int sum = 10 + 5;

The lexical analyzer would produce a token stream like this:

Token TypeToken Value

Role in Compilation

The sequence of tokens produced by the lexical analyzer serves as input to the next phase of compilation, namely, syntax analysis (parsing). The parser verifies that the token sequence adheres to the grammatical structure of the programming language.


Lexical analysis plays a crucial role in compilation:

  • Foundation: It sets the stage by making source code understandable to later compiler phases.
  • Error Detection: Early detection of syntax errors at the lexical level improves compiler efficiency and robustness.
  • Simplification: Breaks down the complex source code into simpler tokens, making parsing more manageable.