In Mat 5:17 the word "Law" is Strong's number G3551, which is Law of Moses, it is also H8451, which is the Decalogue or Pentateuch. These are the first five books of the Bible. All of the Law's of God are contained in the first five books and the Hebrew word that describes this is "Torah". Mat 5:18 is given to us directly from Jesus, it tells us that All the Law or Torah is in effect until the Last Day when heaven and earth do pass away. If you truly follow and obey Jesus you keep the Torah.