import re
text = r"<img src='foobar.jpg'>"
my_regex_pattern = r"""((?<=src=["'])|(?<=href=.))(?!(http(s|)(:|%3[Aa])))([0-9A-Za-z%?&#_=+./~])*(?=['"])"""
prefix = r"http://e...content-available-to-author-only...e.com/bar/"
res = re.sub(my_regex_pattern, prefix.replace('\\', '\\\\') + r'\g<0>', text)
print(res)