protected function parseExpression($valueExpression)
    {
        // expressions are not longer than 2 characters
        $checkForExpression = substr($valueExpression, 0, 2);

        if (str_contains($checkForExpression, static::$validExpressions)) {
            foreach (static::$validExpressions as $expression) {
                if (substr($valueExpression, 0, strlen($expression)) === $expression) {
                    $this->expression = $expression;
                    $this->value = $this->transformValue(substr($valueExpression, strlen($expression)));

                    return;
                }
            }
        }

        // default to "equal to"
        $this->expression = '=';
        $this->value = $this->transformValue($valueExpression);
    }