Lex: A Lexical Analyzer Generator for Compiler Development

Learn about Lex, a powerful tool used to generate lexical analyzers for compilers. This guide explains Lex's input (Lex program), output (C code for a lexical analyzer), and how it simplifies the process of transforming source code into a stream of tokens for parsing.

Lex: A Lexical Analyzer Generator

What is Lex?

Lex is a tool used to generate lexical analyzers—programs that transform a stream of characters (source code) into a sequence of tokens. Tokens are the fundamental building blocks used by parsers during the syntax analysis phase of compilation. Lex simplifies lexical analyzer development. It’s often used with YACC (Yet Another Compiler Compiler), a parser generator.

How Lex Works

The Lex tool takes a Lex program (source code defining the lexical analyzer) as input and generates a C program (usually named `lex.yy.c`). This generated C program is then compiled into an executable lexical analyzer. This lexical analyzer is designed to read input, identify tokens according to the rules specified in the lex program, and output these tokens for further processing by the parser.

Lex File Structure

A Lex program has three sections, separated by `%%`:

1. Definitions: Declarations of constants, variables, and regular expressions used in the rules.
2. Rules: The core logic. Each rule consists of a regular expression (pattern) and an action (code to execute if the pattern matches). The lexical analyzer tries to match these patterns in the input. If a pattern is found, the corresponding action is performed. If no patterns match, the analyzer might throw an error or skip the given character.
3. User Subroutines: Helper functions called by the actions defined in the rules section. These can be compiled separately from the main lexical analyzer program.

{definitions}
%%
{rules}
%%
{user subroutines}