Problem: You want to ensure that your developers are using nothing but the purest ASCII characters in the source code. For example, they may be copying text from Microsoft Word into Javadoc comments.

Solution: Use the Checkstyle GenericIllegalRegexp check to prevent anything but pure ASCII characters. Use the following configuration:

    <module name="GenericIllegalRegexp">
      <property name="format" value="[^\x00-\x7F]"/>
      <property name="message" value="Only use ASCII characters."/>

The "magic" is in the regular expression \[^\\x00-\\x7F\], which says to disallow any characters that are not ASCII. This should keep my fourth reader happy. :-)