Viswanath

Recently I came accross a situation where I have to use Lex tools for writing lexical analyzers. Generally these tools are avilable on most of the standard UNIX based operating sustems. But I use Windows Operating system. So, I just wanted to share my experience of installing them, writing sample programs and compiling them and executing them.

 
Let me first start with how to install them on your windows machine...

(1) Download Flex (Free implementations of LEX library), Bison (Free implementation of Bison library) and  Dev-CPP
(2) Install them on your machine (Note: without spaces in the name of the directory)  
(3)Set PATH variable to include bin directories of Flex, Bison, gcc

Now compiling .l and .y files and generating .exe file...

Open your command prompt and  go to directory where you lex and yacc files are located and run below commands

(1) flex  filename.l
           ===> it generates lex.yy.c file
(2) bison filename.y
           ===> it generates y.tab.c and y.tab.h file
(3) gcc lex.yy.c y.tab.c -o test.exe
           ===> it generates executable file (.exe)
Note:  You can run gcc command only with lex.yy.c file also. ex: gcc lex.yy.c -o .exe


Sample lex file (whitespace.l):
 

non_white [^ \t\n]*
%%

{non_white} ECHO;
.  ;
[\n]  ;

%%

int yywrap(void)
{
    return 0;
}

int main(void)
{
    yylex();
    return 0;
}

Sample lex file (lexicalAnalyzer.l):
 

DIGIT [0-9]
ID [a-z][a-z0-9]*
%%
char|int|float|double|main|void|real|printf|for|while|do   printf("keyword=%s\n",yytext);
{DIGIT}   printf("integer %s\n",yytext);
{ID}+     printf("identifier %s\n",yytext);
{DIGIT}+"."{DIGIT}*   printf("floating point number:%s\n",yytext);
[=|!=|>=|<=|<|>|+|-|*|/]    printf("operator %s\n",yytext);
[(]     printf("openparenthesis %s\n",yytext);
[)]    printf("closeparenthesis %s\n",yytext);
[{]    printf("open bbrace is %s\n",yytext);
[}]   printf("close braceis %s\n",yytext);
[;]   printf("eos %s\n",yytext);
[\t\n]+     /*eat up whitesapaces */
["{"|^}\n]*"}"]      /*eat up comments */
["|,|.]         printf("punctaation :%s \n",yytext);
%%

main()
{
yylex();
}
int yywrap()
{
return 1;
}