The python documentation says "it’s highly recommended that you use raw strings for all but the simplest expressions." (https://docs.python.org/3/library/re.html)
So do that with the attached patch generated by
sed -i -e "s/re.search('/re.search(r'/g" $(git grep -l 're.search(')
The warning can be seen in e.g. python3.7:
$ python3.7 -Wd >>> import re; re.search('\s', '') <stdin>:1: DeprecationWarning: invalid escape sequence \s