Lex program to check whether the input is digit or not

Lex is a computer program that generates lexical analyzers. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

Prerequisite: Flex (Fast lexical Analyzer Generator).

Given an input, the task is to check if the input is digit or not.

Examples:

Input:  28
Output: digit

Input: a
Output: not a digit. 

Input: 21ab
Output: not a digit.

Below is the implementation:

/* Lex program to check whether input is digit or not. */
%{ 
#include<stdio.h> 
#include<stdlib.h> 
%} 
/* Rule Section */
%% 
^[0-9]*  printf("digit"); 
^[^0-9]|[0-9]*[a-zA-Z]  printf("not a digit"); 
. ; 
%% 
int main() 
{ 
        // The function that starts the analysis 
    yylex(); 
        return 0; 
} 

Output:

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