// rextester.com

​

//Rextester.Program.Main is the entry point for your code. Don't change it.

//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

​

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

using System.Data;

​

namespace Rextester

{

    public class Program

    {

        public static void Main(string[] args)

        {

        while (true) {

            DataTable dt = new DataTable();

            Console.WriteLine("Enter What Comes Before the Blank (e.g. 1)");

            string beforeBlank = Console.ReadLine();

            Console.WriteLine("Enter What Comes After the Blank (e.g. =12)");

            string afterBlank = Console.ReadLine();

            Console.WriteLine("Enter Allowd Characters for Filling the Blank (e.g. 51+*)");

            string input = Console.ReadLine();

            for (int n=0; n<=input.Length; n++) {

              foreach (string s in GetAllPermutations(input,n)) {

                try {  

                    if (dt.Compute(beforeBlank + s.ToString() + afterBlank,"").ToString() == "True") Console.WriteLine(s.ToString());     

                }catch(Exception e){

                }  

              }

            }

​

        }   

        }

            static ICollection<string> result;

​

    public static ICollection<string> GetAllPermutations(string str, int outputLength)

    {

        result = new List<string>();

        MakePermutations(str.ToCharArray(), string.Empty, outputLength);

        return result;

    }

​

    private static void MakePermutations(

       char[] possibleArray,//all chars extracted from input

       string permutation,

       int outputLength)//the length of output 

    {

         if (permutation.Length < outputLength)

         {

             for (int i = 0; i < possibleArray.Length; i++)

             {

                 var tempList = possibleArray.ToList<char>();

                 tempList.RemoveAt(i);

                 MakePermutations(tempList.ToArray(), 

                      string.Concat(permutation, possibleArray[i]), outputLength);

             }

         }

         else if (!result.Contains(permutation))

            result.Add(permutation);

    }

    }

}