Skip to content
Related Articles

Related Articles

Save Article
Improve Article
Save Article
Like Article

Lex program to check if a Date is valid or not

  • Last Updated : 03 May, 2019

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

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.

Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry ready.


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


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

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


/* Lex program to check if a date is valid or not */
   /* Definition section */
   int i=0, yr=0, valid=0;
/* Rule Section */
            \/([1-2][0-9][0-9][-0-9]) {valid=1;}
         \/([1-2][0-9][0-9][0-9]) {valid=1;}
            \/([1-2][0-9][0-9][0-9]) {valid=1;}
     { while(yytext[i]!='/')i++; i++;
// driver code 
  yyin=fopen("vpn.txt", "r");
  if(valid==1) printf("It is a valid date\n");
  else printf("It is not a valid date\n");
 int yywrap()
 return 1;


My Personal Notes arrow_drop_up
Recommended Articles
Page :