using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        string formula = "AB+FB+100";

        var length = formula.Length;

        List<string> variables = new List<string>();

        List<char> operators = new List<char>

        {

        '+', '-', '*', '/', ')', '(', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'

        }

​

        ;

        int count = 0;

        string character = string.Empty;

        for (int i = 0; i < length; i++)

        {

            if (!operators.Contains(formula[i]))

                character += formula[i];

            else

            {

                if (!string.IsNullOrWhiteSpace(character))

                    variables.Add(character);

                character = string.Empty;

                count = i;

            }

        }

​

        if (!string.IsNullOrWhiteSpace(character))

            variables.Add(character);

        Console.WriteLine("Correct Behaviour Formula : AB+FB+100");

        foreach (var item in variables)

            Console.WriteLine(item);

        Console.WriteLine();

        Console.WriteLine();

        /*****************************************************/

        /*****************************************************/

        /*****************************************************/

        /*****************************************************/

        formula = "AB1+FB1+100";

        length = formula.Length;

        variables = new List<string>();

        count = 0;

        character = string.Empty;

        for (int i = 0; i < length; i++)

        {

            if (!operators.Contains(formula[i]))

                character += formula[i];

            else

            {

                if (!string.IsNullOrWhiteSpace(character))

                    variables.Add(character);

                character = string.Empty;

                count = i;

            }

        }

​

        if (!string.IsNullOrWhiteSpace(character))

            variables.Add(character);

        Console.WriteLine("InCorrect Behaviour Formula : AB1+FB1+100");

        foreach (var item in variables)

            Console.WriteLine(item);

    }

}