Lex Tutorial

Phases of Compilation

Lex

• Lex is a program that generates lexical analyzer. It is used with YACC parser generator.
• The lexical analyzer is a program that transforms an input stream into a sequence of tokens.
• It reads the input stream and produces the source code as output through implementing the lexical analyzer in the C program.

The function of Lex is as follows:

• Firstly lexical analyzer creates a program lex.1 in the Lex language. Then Lex compiler runs the lex.1 program and produces a C program lex.yy.c.
• Finally C compiler runs the lex.yy.c program and produces an object program a.out.
• a.out is lexical analyzer that transforms an input stream into a sequence of tokens.
  

The Lex Format

A Lex program is separated into three sections by %% delimiters. The format of Lex source is as follows:

{ definitions }
%%

{ rules }   

%%
{ user subroutines }

• Definitions include declarations of constant, variable and regular definitions.
• Rules define the statement of form p1 {action1} p2 {action2}....pn {action}. Where pi describes the regular expression and action1 describes the actions what action the lexical analyzer should take when pattern pi matches a lexeme.
• User subroutines are auxiliary procedures needed by the actions. The subroutine can be loaded with the lexical analyzer and compiled separately.