1. A mathematical expression input apparatus comprising: a display unit; and a processor which comprises a CPU and which executes: a mathematical expression display process of displaying an input mathematical expression on the display unit, a determination process of determining whether or not a number or an operator included in the mathematical expression displayed by the mathematical expression display process is designated by a user, a selection process of, when it is determined that the operator included in the displayed mathematical expression is designated by the user, displaying plural mathematical expression candidates with different ranges of incorporation of the designated operator to urge the user to select one of the displayed mathematical expression candidates, and modifying the displayed mathematical expression into a form corresponding to the selected mathematical expression candidate, a replacement process of, when it is determined that the number included in the mathematical expression is designated by the user, allowing the user to re-input said number, without displaying the plural mathematical expression candidates, and replacing the designated number in the mathematical expression with the re-input number, a window display process of, when the mathematical expression is being displayed by the mathematical expression display process, displaying operators included in the mathematical expression in an order in a window, and an order change process of, when the order of the operators displayed in the window is changed in accordance with an operation by the user, modifying the mathematical expression in accordance with the changed order of the operators.