Check that each character is appropriate

Currently, the program just checks to see if the user input a seven-character string; if the user input isn't seven-characters, it shows an error message and asks them to repeat again. However, we want to be more thorough in making sure the format is correct.

Specifically, the input needs to satisfy **all** of the following:

1. It must be exactly seven characters long
2. The first character must be `#`
3. Each of the following six characters must be either a digit (`0` through `9`, inclusive) or a letter (`a` through `f`, inclusive and upper/lower case are both acceptable)

When any of those requirements are not met, the error message should display and the user should be asked again for the input.