Lex program to check if a Date is valid or not

Problem: Write a Lex program to check if a date is valid or not.

Explanation:
Flex (Fast lexical Analyzer Generator) is a tool/computer program for generating lexical analyzers (scanners or lexers) written by Vern Paxson in C around 1987. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language. The function yylex() is the main flex function which runs the Rule Section.

Note: Format of Date is DD/MM/YYYY.

Examples:

Input: 02/05/2019
Output: It is a valid date

Input: 05/20/2019
Output: It is not a valid date

Implementation:

/* Lex program to check if a date is valid or not */
  
%{ 
   /* Definition section */
   #include<stdio.h> 
   int i=0, yr=0, valid=0; 
%} 
  
/* Rule Section */
%% 
([0-2][0-9]|[3][0-1])\/((0(1|3|5|7|8))|(10|12)) 
            \/([1-2][0-9][0-9][-0-9]) {valid=1;} 
  
([0-2][0-9]|30)\/((0(4|6|9))|11) 
         \/([1-2][0-9][0-9][0-9]) {valid=1;} 
  
([0-1][0-9]|2[0-8])\/02 
            \/([1-2][0-9][0-9][0-9]) {valid=1;} 
  
29\/02\/([1-2][0-9][0-9][0-9])  
     { while(yytext[i]!='/')i++; i++; 
       while(yytext[i]!='/')i++;i++; 
       while(i<yyleng)yr=(10*yr)+(yytext[i++]-'0');  
       if(yr%4==0||(yr%100==0&&yr%400!=0))valid=1;} 
  
%% 
  
// driver code  
main() 
{ 
  yyin=fopen("vpn.txt", "r"); 
  yylex(); 
  if(valid==1) printf("It is a valid date\n"); 
  else printf("It is not a valid date\n"); 
} 
 int yywrap() 
{ 
 return 1; 
} 

Output:

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