import re


def trim_xml_html_tags(data_str):
    """
    Trim all HTML/XML tags and replace the special "panel" tag with a new line
    :param data_str: input data string to trim
    :return: Trimmed string

    >>> trim_xml_html_tags('')
    ''
    >>> trim_xml_html_tags(u'')
    ''
    >>> trim_xml_html_tags(u'hello world')
    'hello world'
    >>> trim_xml_html_tags('hello world')
    'hello world'
    >>> trim_xml_html_tags('<h1>hello world</h1>')
    'hello world'
    >>> trim_xml_html_tags('<div><h1>hello world!</h1></div><div class="panel panel-default">:)')
    'hello world!\\n:)'
    """
    return re.sub('<[^<]+?>', '', data_str.replace('</div><div class="panel panel-default">', "\n"))