If you've written many full-blown applications in Java, especially ones in which input is accepted from a Web form or a Swing GUI, you've probably spent some time writing validation code -- that ...

Input validation is used to ensure that only whitelisted input is accepted. In this case, a regex is defined to accept only known good characters that are suitable for use in a comments field.

At this point, the Scanner still returns a String, although the String contains only one character. To complete the use case, you must convert this one-character String into a single Java char with ...