You could use .find("is"), it would return position of "is" in the string
or use .start() from re
>>> re.search("is", String).start()
2
Actually its match "is" from "This"
If you need to match per word, you should use \b before and after "is", \b is the word boundary.
>>> re.search(r"\bis\b", String).start()
5
>>>
for more info about python regular expressions, docs here