lex.md

General structure of a lex file

Before we start, let's understand that this sample.l lex file will be used by the lex command to generate a C code. This C code will be able to take a stream of text as input, and match that stream of text against the regular expressions that you have defined in the file. If a match is found, then some action can be taken (which is again, defined by you).

The generated C file will have the name lex.yy.c

The General Format of a Lex File consists of three sections:

1. Definitions:

Anything in this part is simply copied to lex.yy.c file. This part generally includes #define, #include etc.

2. Rules:

This is the cool part. Where you define regular epxressions and specify what actions will be taken when a match is found. These actions could be simple, or could be very complex functins. If the action is a function, then you need to define that function in section 3 of the file.

3. User Subroutines

These are C functions that you might want to put in that lex.yy.c file.

A sample lex file is explained below

Definition Part

%{
#include<stdio.h>
#include<math.h>
#include "cali.tab.h"
int yywrap(void);
%}

Rules part

%%
[0-9]+ {
 yylval.dval=atoi(yytext);
 return NUMBER;
 }
[t];
n {   

	return 0;
  }
\n {
	printf("this is new line\n");
	}
. {return yytext[0];}
%%

User Subroutines part

int yywrap(){
	  printf("End of parsing \n");
  }
void yyerror(const char *str)
{
 printf("n Invalid Character...");
}
int main(){
 printf("Enter Expression => ");

 yyparse();
 return(0);
}

Just put all the three parts above in a file and save it as something.l