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:

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/* 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; }

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Output:

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