2X + 5Y + z^3

int thirdValue
string temp;
temp = Regex.Match(variables[3], @"d+").Value
thirdValue = int.Parse(temp);

string temp = textBox1.Text;
        char[] arra = temp.ToCharArray();
        int total = 0;
        foreach (char t in arra)
        {
            if (char.IsDigit(t))
            {
                total += int.Parse(t + "");
            }
        }
        textBox1.Text = total.ToString();

string temp;
temp = Regex.Matches(textBox1.Text, @"d+", RegexOptions.IgnoreCase)[2].Value;
int thirdValue = int.Parse(temp);

static class Program
{
    static void Main()
    {

        Console.WriteLine("2X + 65Y + z^3".GetNumbersFromString().Sum());

        Console.ReadLine();
    }


    static IEnumerable<int> GetNumbersFromString(this string input)
    {
        StringBuilder number = new StringBuilder();
        foreach (char ch in input)
        {
            if (char.IsDigit(ch))
            {
                number.Append(ch);
            }
            else if (number.Length > 0)
            {
                yield return int.Parse(number.ToString());
                number.Clear();
            }
        }
        yield return int.Parse(number.ToString());
    }
}

Regex.Match(variables[3], @"d+").Value

if (Regex.IsMatch(variables[3], @"d+")){
   temp = Regex.Match(variables[3], @"d+").Value
}