Check that each character is appropriate

[kbuffardi]

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.