Lex program to check whether an year is a leap year or not

Problem: Write a Lex program to check whether an year is a leap year or not.

Explanation:
Lex is a computer program that generates lexical analyzers and was written by Mike Lesk and Eric Schmidt. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

Note: Leap years are the years that are divisible by 400 or are divisible by 4 and not divisible by 100. So 2016, 2020 etc are leap years while 2017 and 2019 are not.

Examples:

Input: 2016 
Output: leap year

Input: 2017
Output: not a leap year 

Implementation:

/*Lex program to check whether an year is a leap year or not*/

%{
 void check(char *);
%}

/*Rule Section*/
 %%
[0-9]    ;
[0-9][0-9]    ;
[0-9][0-9][0-9]     ;
[0-9][0-9][0-9][0-9]    { printf("%s", yytext);check(yytext); }
[0-9][0-9][0-9][0-9][0-9]+ ;
 %%

// driver program
int main()
{
    extern FILE *yyin;
    yyin=fopen("num", "r");

    // The function that starts the analysis
    yylex();
    return 0;

}

void check(char *a)
{
    int x=0, i;

    for(i=0;i<4;i++)
        x=x*10+(a[i]-'0');

    if(x%400==0)
        printf("\tleap year\n");

    else if(x%4==0&&x%100!=0)
        printf("\tleap year\n");

    else
        printf("\tnot a leap year\n");
}

Output: