 problems is for the code to issue a
        user-friendly message at run time.  It should <b>verify the user
        input</b> using the
        <a href="https://checkerframework.org/api/org/checkerframework/checker/regex/util/RegexUtil.html#isRegex(java.lang.String)"><code>RegexUtil.isRegex(String, int)</code></a> method.
        If the string is not a valid regular
        expression, it should <b>print an error message and halt</b>.
        If it <em>is</em> a valid regular expression, perform as before