Lexical Analysis
Lexical analysis is the first phase of compiling or interpreting source code, in which a scanner (or lexer) reads raw characters and groups them into meaningful tokens such as keywords, identifiers, literals, and operators.
Definition
Lexical analysis is the first phase of compiling or interpreting source code, in which a scanner (or lexer) reads raw characters and groups them into meaningful tokens such as keywords, identifiers, literals, and operators.
Overview
Before a compiler can understand what a program means, it must first break the raw stream of characters into recognizable pieces. Lexical analysis performs this job: a lexer scans the source text left to right and matches sequences of characters against patterns that define each token type, discarding whitespace and comments along the way. For example, the text `total = price * 1.08` is broken into the tokens `total` (identifier), `=` (operator), `price` (identifier), `*` (operator), and `1.08` (numeric literal). The patterns used to recognize tokens are typically expressed using Regular Expression Theory, and lexers are commonly implemented as Finite State Machines that transition between states as each character is read, recognizing a complete token once no further characters can extend a valid match. Tools such as Lex and Flex can automatically generate a lexer from a set of token pattern definitions, a technique that traces back to foundational work in automata theory. The resulting stream of tokens is passed to the parser, which arranges them according to the language's grammar to build an Abstract Syntax Tree. Lexical analysis is deliberately kept separate from parsing because pattern matching for tokens (a 'regular' language problem) is computationally simpler than parsing nested grammatical structure (a 'context-free' language problem), and splitting the two responsibilities keeps each stage of Compiler Design easier to implement, test, and optimize.
Key Concepts
- Converts a raw character stream into a sequence of typed tokens
- Discards insignificant whitespace and comments during scanning
- Token patterns are typically defined using regular expressions
- Commonly implemented internally as a finite state machine
- Can be auto-generated from pattern definitions using tools like Lex or Flex
- Detects certain classes of errors early, such as invalid characters
- Precedes parsing, which builds structure from the resulting token stream
- Kept as a distinct phase because regular-language matching is simpler than context-free parsing
Use Cases
Frequently Asked Questions
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