Compiler Design and Construction - Old Questions

3.  Discuss the specification of lexical analyzer generator Lex.

Lexical Analyzer Generator (Lex/Flex) systematically translate regular definitions into C source code for efficient scanning. Generated code is easy to integrate in C applications. Flex is a latter version of lex.

• Firstly, specification of a lexical analyzer is prepared by creating a program lex.l in Lex/Flex
• lex.l is run into lex compiler to produce a C program lex.yy.c
• lex.yy.c consists of the tabular representation of lex.l, together with a standard routine that uses the table to recognize lexeme.
• Lex.yy.c is run through C compiler to produce the object program a.out which is a lexical analyzer that input into tokens.

A  lex/flex specification consists of three parts:

1. regular definitions, C declarations in %{ %}

        %%

2. translation rules

        %%

3. user-defined auxiliary procedures

The translation rules are of the form:

    p1     { action1 }

    p2     { action2 }

    …

    pn     { actionn }

Lex/flex regular definitions are of the form:

       name definition

E.g. Digit   [0-9]

Example of lex specification

%{

    #include <stdio.h>

%}

%%

[0-9]+   { printf(“%s\\n”, yytext); }

. | \\n     {  }

%%

main()

{ yylex();

}