how to change what is under the diagonals in a matrix into Zeros

how do i turn what is under the diagonal on a matrix into zeros. for example i cant figure out how to do the following.

[1 2 3,4 5 6,7 8 9]

into

[1 2 3,0 5 6,0 0 9]

Answer 1

[Matt J]

See the TRIU command.