LEX Code that accepts the string with 0 only

Last Updated : 23 Jul, 2021

In this article, we will discuss the overview of the LEX Code that accepts the string with 0 only. And will implement with LEX code also, we will understand the approach. Let’s discuss it one by one.

Problem Overview :
LEX Code that accepts the string with 0 only.

Example –

Input  : 00
Output : Accepted

Input  : 1000
Output : Invalid

Input  : 23ab
Output : Invalid

Input  : ab345
Output : Invalid

Input  : 00000
Output : Accepted

Approach :
LEX provides us with an INITIAL state by default. So to make a DFA, use this as the initial state of the DFA. We define two more states: A, and DEAD, where the DEAD state would be used if encountering a wrong or invalid input. When the user inputs an invalid character, move to DEAD state, and then print “Invalid”. If the input string ends at A then display the message “Accepted”. Else if the input string ends at state INITIAL then displays the message “Not Accepted”.

Note –
To compile the lex program we need to have a Unix system that has flex installed into it. Then we need to save the file with the .l extension.

Example :

filename.l

Then after saving the program closes the lex file and then open the terminal and write the following commands as follows.

lex filename.l
cc lex.yy.c
./a.out

LEX Code :

C

%{ 
%} 
   
%s A DEAD 
   
// logic for dead state   
%% 
<INITIAL>0 BEGIN A; 
<INITIAL>[^0\n] BEGIN DEAD; 
<INITIAL>\n BEGIN INITIAL; {printf("Not Accepted\n");} 
  
// Initial node - Accepted 
<A>0 BEGIN A; 
<A>[^0\n] BEGIN DEAD; 
<A>\n BEGIN INITIAL; {printf("Accepted\n");} 
  
// Invalid Case 
<DEAD>[^\n] BEGIN DEAD; 
<DEAD>\n BEGIN INITIAL; {printf("Invalid\n");} 
   
%% 
  
    
// Method - yywrap   
int yywrap() 
{ 
return 1; 
}   
  
// main method 
int main() 
{ 
   printf("Enter String\n"); 
  // called yylex method 
   yylex(); 
return 0; 
}