* Default constructor.
     */
    public NotificationHelper() {
        // Default constructor
    }

    private static final Logger logger = LogManager.getLogger(NotificationHelper.class);

    /** Line feed character for message formatting. */
    protected static final char LF = '\n';

    /**
     * Sends notifications to configured platforms.
     *
     * @param cardView the card view for the notification

     *
     * This method removes characters that are not allowed in XML documents
     * according to the XML 1.0 specification. Valid characters include:
     * - Tab (0x9)
     * - Line feed (0xA)
     * - Carriage return (0xD)
     * - Any character between 0x20 and 0xD7FF
     * - Any character between 0xE000 and 0xFFFD
     * - Any character between 0x10000 and 0x10FFFF
     *

## Advanced description from docstring { #advanced-description-from-docstring }

You can limit the lines used from the docstring of a *path operation function* for OpenAPI.

Adding an `\f` (an escaped "form feed" character) causes **FastAPI** to truncate the output used for OpenAPI at this point.

It won't show up in the documentation, but other tools (such as Sphinx) will be able to use the rest.

   * the XML specification.
   *
   * <p>This escaper does not escape non-ASCII characters to their numeric character references
   * (NCR). However, horizontal tab {@code '\t'}, line feed {@code '\n'} and carriage return {@code
   * '\r'} are escaped to a corresponding NCR {@code "&#x9;"}, {@code "&#xA;"}, and {@code "&#xD;"}
   * respectively. Any other non-ASCII characters appearing in the input will be preserved in the

            protected PathMappingHelper getPathMappingHelper() {
                return new PathMappingHelper();
            }
        };
        fessXpathTransformer.init();
        fessXpathTransformer.convertUrlMap.put("feed:", "http:");

        List<RequestData> urlList = new ArrayList<>();

        urlList = fessXpathTransformer.convertChildUrlList(urlList);
        assertEquals(0, urlList.size());

        urlList.clear();

 *       {@code Number}, and primitive varargs are supported.
 *   <li>Or, to avoid storing up all the data first, create a {@link StatsAccumulator} instance,
 *       feed values to it as you get them, then call {@link StatsAccumulator#snapshot}.
 * </ul>
 *
 * <p>Static convenience methods called {@code meanOf} are also provided for users who wish to
 * calculate <i>only</i> the mean.
 *

</p>

<p>
After a backslash, certain single-character escapes represent special values:
</p>

<pre class="grammar">
\a   U+0007 alert or bell
\b   U+0008 backspace
\f   U+000C form feed
\n   U+000A line feed or newline
\r   U+000D carriage return
\t   U+0009 horizontal tab
\v   U+000B vertical tab
\\   U+005C backslash
\'   U+0027 single quote  (valid escape only within rune literals)

* `$`: ends there, doesn't have any more characters after `fixedquery`.

If you feel lost with all these **"regular expression"** ideas, don't worry. They are a hard topic for many people. You can still do a lot of stuff without needing regular expressions yet.

Now you know that whenever you need them you can use them in **FastAPI**.

# Environment Variables { #environment-variables }

/// tip

If you already know what "environment variables" are and how to use them, feel free to skip this.

///

An environment variable (also known as "**env var**") is a variable that lives **outside** of the Python code, in the **operating system**, and could be read by your Python code (or by other programs as well).

///

## Write the callback documentation code { #write-the-callback-documentation-code }

This code won't be executed in your app, we only need it to *document* how that *external API* should look like.

But, you already know how to easily create automatic documentation for an API with **FastAPI**.