Lexical Analysis in Compiler Design: Transforming Source Code into Tokens
Understand lexical analysis (scanning), the initial phase of compilation. This guide explains how lexical analyzers transform raw source code into a stream of tokens, the building blocks for subsequent syntax analysis (parsing), and techniques like regular expressions and finite automata used in lexical analysis.
Lexical Analysis in Compiler Design
What is Lexical Analysis?
Lexical analysis (or scanning) is the first phase of compilation. It involves reading the source code and breaking it down into a stream of tokens—meaningful units of the programming language, such as keywords, identifiers, operators, numbers, and punctuation. This is essential because it transforms the raw input (a sequence of characters) into a more structured representation, which the next stage of the compiler (syntax analysis or parsing) can then easily work with. The lexical analyzer, often implemented using tools like Lex, performs this task efficiently, and it also helps to identify errors (lexical errors) in the source code itself.
Key Terms in Lexical Analysis
- Tokens: Meaningful units in the language (keywords, identifiers, etc.).
- Lexemes: Sequences of characters in the source code that match a token pattern.
- Patterns: Rules defining the structure of tokens (regular expressions).
Example Tokens, Lexemes, and Patterns
| Token | Lexeme | Pattern |
|---|---|---|
| Keyword | while |
w h i l e (sequence of characters) |
| Relational Operator | <= |
< | > | <= | >= | == | != |
| Integer | 123 |
Digit{1,} (one or more digits) |
| String | "Hello" |
" [^"]* " (characters within double quotes) |
| Identifier | myVariable |
[a-zA-Z_][a-zA-Z0-9_]* (letter followed by letters, digits, or underscores) |
| Punctuation | ; |
[;, . , : , ( , )] (semicolon, comma, period, etc.) |
Steps in Lexical Analysis
- Remove comments and whitespace.
- Break the input into lexemes.
- Classify lexemes into tokens.
- Detect lexical errors.
- Pass tokens to the syntax analyzer.
- Add identifier entries to the symbol table.
Input Buffering
Lexical analyzers use input buffering to efficiently handle input. Methods include:
- One-buffer scheme: Uses two pointers to manage a single buffer.
- Two-buffer scheme: Uses two buffers to improve efficiency (one buffer is loaded while the other is processed).
Patterns and Regular Grammars
Lexical analyzers use regular expressions (defined using regular grammars) to identify lexemes. Examples include:
- Numbers: `Digit{1,}` (one or more digits)
- Delimiters: `[ \t\n]` (whitespace, tab, newline)
- Identifiers: `[a-zA-Z_][a-zA-Z0-9_]*`
Finite Automata and Lexical Analysis
Finite automata (FAs) are used to recognize valid token patterns. The lexical analyzer creates a transition diagram to represent the regular grammar rules. If the lexeme reaches the final state, the lexeme is a valid token.
Handling Lexical Errors
When a lexeme doesn’t match any valid token, the lexical analyzer goes into “panic mode,” attempting to recover by performing transformations such as deleting characters, replacing characters, transposing characters, or inserting characters. This allows the lexical analyzer to continue processing even when it encounters errors in the input.