import re
print(
re.sub(r'#[^#]+#',
r'<>\g<0></>',
r'Эта строка #test# написана 19.01.2018, #x# а могла бы и 01.09.2017')
)
aW1wb3J0IHJlCnByaW50KAoJcmUuc3ViKHInI1teI10rIycsIAogICAgICAgICAgIHInPD5cZzwwPjwvPicsIAogICAgICAgICAgIHIn0K3RgtCwINGB0YLRgNC+0LrQsCAjdGVzdCMg0L3QsNC/0LjRgdCw0L3QsCAxOS4wMS4yMDE4LCAgI3gjINCwINC80L7Qs9C70LAg0LHRiyDQuCAwMS4wOS4yMDE3JykKKQ==