Validate a Signed Decimal and Exponent Spelling

### Problem Check whether the entire string `spelling` has the syntax of a valid number under the grammar below. Return a boolean result. The main numeric part may begin with one optional sign, `+` or `-`. After that sign, it must have one of these forms: - One or more decimal digits, with no dot. - One or more digits followed by a dot, optionally followed by more digits. - A dot followed by one or more digits. An optional exponent may follow the main part. It consists of `e` or `E`, then one optional sign, then one or more decimal digits. A decimal point is not allowed inside the exponent. All characters must belong to this complete structure. Leading zeroes are permitted. The task checks syntax and returns no converted numeric value. ### Examples **Example 1** ```text Input: spelling = "-.625E+2" Output: true ``` The main part is a signed decimal with digits after its dot, and `E+2` is a valid exponent. **Example 2** ```text Input: spelling = "7e-3.2" Output: false ``` The exponent must contain an integer spelling; the dot makes this exponent invalid. ### Constraints - `1 <= spelling.length <= 20` - Characters may be uppercase or lowercase English letters, decimal digits, `+`, `-`, or `.`.