re.sub example

import re


def regex_replace(string, replace):
    regex = re.compile(r'(?P<start><!-- start Menu -->)(?P<middle>[\w\d\s]+)(?P<end><!-- end Menu -->)')
    output = r'\1' + replace + r'\3'
    return regex.sub(output, string)

regex_replace('<!-- start Menu -->  und  <!-- end Menu -->', 'Hello World')
# '<!-- start Menu -->Hello World<!-- end Menu -->'