public void StringExpressionChecker_OneDigit_ShouldPass()
        {
            // Arrange
            const string expression = "4";

            // Act
            bool isValid = StringExpressionChecker.IsValid(expression);

            // Assert
            Assert.IsTrue(isValid);
        }
        public void StringExpressionChecker_DivisionByZero_ShouldFail()
        {
            // Arrange
            const string expression = "4+4/0+2";

            // Act
            bool isValid = StringExpressionChecker.IsValid(expression);

            // Assert
            Assert.IsFalse(isValid);
        }
        public void StringExpressionChecker_TwoMathOperationsNextToEachOther_ShouldFail()
        {
            // Arrange
            const string expression = "4++4";

            // Act
            bool isValid = StringExpressionChecker.IsValid(expression);

            // Assert
            Assert.IsFalse(isValid);
        }
        public void StringExpressionChecker_LastIsMathOperation_ShouldFail()
        {
            // Arrange
            const string expression = "4+4+";

            // Act
            bool isValid = StringExpressionChecker.IsValid(expression);

            // Assert
            Assert.IsFalse(isValid);
        }
        public void StringExpressionChecker_CorrectExpression_ShouldPass()
        {
            // Arrange
            const string expression = "4+4/2";

            // Act
            bool isValid = StringExpressionChecker.IsValid(expression);

            // Assert
            Assert.IsTrue(isValid);
        }