Implementation of Lexical Analyser
Duration: 4 min
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The lecture provides a comprehensive overview of the implementation phase in compiler design, specifically focusing on lexical analysis. It begins by defining the lexical grammar as a set of rules that establish the lexical syntax of a programming language. The instructor emphasizes that this syntax is typically modeled as a regular language, where grammar rules are expressed as regular expressions. These expressions define the valid sets of character sequences, known as lexemes, which correspond to specific tokens. A key theoretical point is made by equating Regular Language (RL), Regular Grammar (RG), Regular Expression (RE), and Finite Automata (FA). The session then transitions to the practical Design of Lexical Analyser, contrasting Hand Coding with tools like Lex Tool. The hand-coding process involves constructing regular expressions and converting them into finite automata. A concrete example is worked through involving a password validation rule using the regular expression A(A+S+D)^7 (A+S+D+e)^7. The instructor analyzes this to determine the specific character constraints for a valid password token. Finally, the lecture concludes with a structural overview of the lexical analyzer's role, showing how it interacts with the parser and symbol table to identify tokens and manage compilation errors.
Chapters
0:00 – 2:00 00:00-02:00
The instructor starts with a slide titled Implementation, outlining the theoretical basis for lexical analysis. He explains that a programming language specification includes a lexical grammar which defines the lexical syntax. He underlines these terms to emphasize their importance. The slide further states that lexical syntax is usually a regular language, with grammar rules consisting of regular expressions. These expressions define the set of possible character sequences, or lexemes, for a token. The instructor writes RL = RG = RE = FA on the slide, visually demonstrating the equivalence between Regular Language, Regular Grammar, Regular Expression, and Finite Automata. This section establishes the mathematical foundation for how a lexer recognizes strings and produces tokens.
2:00 – 4:22 02:00-04:22
The lecture moves to the Design of Lexical Analyser, presenting two approaches: Hand Coding and Lex Tool. Under Hand Coding, the instructor lists the steps: Construct the Regular Expression and Convert the Regular Expression into Finite Automata. A specific question is posed to identify the meaning of a rule implemented by a lexer for password tokens: Regular Expression -> A(A+S+D)^7 (A+S+D+e)^7. The instructor breaks down this expression, explaining that it starts with a letter (A), followed by 7 alphanumeric characters, and then 7 more alphanumeric characters or an empty string, totaling a 15-character password. He underlines parts of the regex to illustrate the breakdown. The segment ends with a slide defining token components like letter, digit, identifier, and number, followed by a diagram showing the Lexical Analyzer receiving the source program, interacting with the Symbol Table, and sending tokens to the Parser.
The lesson progresses from theoretical definitions of lexical syntax and regular languages to practical design methods for lexical analyzers. By analyzing a specific password validation regex, the instructor demonstrates how abstract regular expressions translate into concrete token recognition rules. The final diagram contextualizes the lexer within the broader compiler architecture, highlighting its critical role in bridging source code and parsing.