import re


def bold(corpus):
    """
    >>> text = '**bold**' / '__bold__'
    >>> bold(text)
    '<strong>bold</strong>'
    """
    underscore = re.sub(r'\_\_([^\n]+)\_\_', r'<strong>\g<1></strong>', corpus)
    return re.sub(r'\*\*([^\n]+)\*\*', r'<strong>\g<1></strong>', underscore)