Lex program to recognize valid arithmetic expression and identify the identifiers and operators

Problem: Write a Lex program to recognize valid arithmetic expression and identify the identifiers and operators.

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.

Examples:

Input: a+b*c
Output: valid expression
        the operators are :
        + 
        * 
        the identifiers  are :
        a 
        b 
        c

Input: a+b-
Output: invalid expression
 
Input: (a*b)
Output: valid expression
        the operators are :
        * 
        the identifiers are :
        a 
        b 

Input: (a+b-
Output: invalid expression 

Implementation:

/* Lex program to recognize valid arithmetic expression
        and identify the identifiers and operators */
%{
#include <stdio.h>
#include <string.h>
    int operators_count = 0, operands_count = 0, valid = 1, top = -1, l = 0, j = 0;
    char operands[10][10], operators[10][10], stack[100];
%}
%%
"(" {
    top++;
    stack[top] = '(';
}
"{" {
    top++;
    stack[top] = '{';
}
"[" {
    top++;
    stack[top] = '[';
}
")" {
    if (stack[top] != '(') {
        valid = 0;
    }
    else if(operands_count>0 && (operands_count-operators_count)!=1){
        valid=0;
    }
    else{
        top--;
        operands_count=1;
        operators_count=0;
    }
}
"}" {
    if (stack[top] != '{') {
        valid = 0;
    }
    else if(operands_count>0 && (operands_count-operators_count)!=1){
        valid=0;
    }
    else{
        top--;
        operands_count=1;
        operators_count=0;
    }
}
"]" {
    if (stack[top] != '[') {
        valid = 0;
    }
    else if(operands_count>0 && (operands_count-operators_count)!=1){
        valid=0;
    }
    else{
        top--;
        operands_count=1;
        operators_count=0;
    }

}
"+"|"-"|"*"|"/" {
    operators_count++;
    strcpy(operators[l], yytext);
    l++;
}
[0-9]+|[a-zA-Z][a-zA-Z0-9_]* {
    operands_count++;
    strcpy(operands[j], yytext);
    j++;
}
%%


int yywrap()
{
    return 1;
}
int main()
{
    int k;
    printf("Enter the arithmetic expression: ");
    yylex();

    if (valid == 1 && top == -1) {
        printf("\nValid Expression\n");
    }
    else
        printf("\nInvalid Expression\n");

    return 0;
}

Output: