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Lexical Analysis

Demystifying Lexical Analysis: A Beginner's Guide to Building a Compiler

A lexical analyzer, also known as a lexer or scanner, is an essential component of a compiler.

Its main purpose is to convert the source code of a programming language into a sequence of tokens that can be processed by the parser.

A lexer is responsible for recognizing the lexemes or basic building blocks of the language, such as keywords, identifiers, literals, operators, and punctuators.

In this article, we will discuss the design of a lexical analyzer for a sample language and the steps involved in implementing it using the LEX tool.

Sample Language Specification

To illustrate the design of a lexical analyzer, we will define a simple programming language called SAMPLE, which has the following characteristics:

  1. The language supports only integer data type.
  2. The language has the following keywords: IF, ELSE, WHILE, DO, and INT.
  3. The language uses the following operators: +, -, *, /, %, <, <=, >, >=, ==, and !=.
  4. The language uses the following punctuators: ;, ,, (, and ).
  5. The language allows identifiers to start with a letter and contain letters and digits.
  6. The language ignores white space and comments, which start with /* and end with */.

Design of Lexical Analyzer

The design of a lexical analyzer involves several steps, including defining the token types, specifying the regular expressions for each token type, and generating the lexer code using a tool such as LEX. Let us discuss these steps in detail.

  1. Defining Token Types

The first step in designing a lexical analyzer is to define the token types for the language. In our example, we have the following token types:

Keyword: IF, ELSE, WHILE, DO, INT
Identifier: A sequence of letters and digits starting with a letter
Integer Literal: A sequence of digits
Operator: +, -, *, /, %, <, <=, >, >=, ==, !=
Punctuator: ;, ,, (, )

  1. Specifying Regular Expressions

The next step is to specify the regular expressions for each token type. A regular expression is a pattern that matches a set of strings, and it is used to define the lexical structure of the language. In our example, we have the following regular expressions:

Keyword: IF|ELSE|WHILE|DO|INT
Identifier: [a-zA-Z][a-zA-Z0-9]*
Integer Literal: [0-9]+
Operator: +|-|*|/|%|<|<=|>|>=|==|!=
Punctuator: ;|,|∣

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