import re
def m(r, s):
    mtc = re.match(r, s)
    if mtc:
        return s
    return "false"
def italic(s):
    if  m(r'\*{2}[^\*]+\*{2}', s) == 'false':
        return re.sub(r'\*([^*]+)\*', r'<em>\1</em>', s)
    return s
print(italic('my plain text'))
print(italic('*my italic text*'))
print(italic('**now it is bold**'))