As the previous commenter noted, you have not said what you mean by "word" and "sentence." Assuming a sentence is a bunch of things delimited by white space, you could use /(.+)\s+(.+)/. Because matches are greedy the first capture group grabs everything up to the last delimiter, and the second grabs everything after that.

If your sentences contain line breaks you will want the s qualifier on your regular expression. That is, /(.+)\s+(.+)/s.

If you want to take punctuation into account, that is much more complicated, since the period/full-stop character can also occur internally (say, in "Dr. Jones performs laparoscopy.")