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:

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/* 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;
}

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