How to typeset block matrices?

I need to typeset a block matrix that is made of a 2X2 structure of blocks such that the (1,1) block and the (2,2) blocks are zero and the (1,2) and the (2,1) blocks are non-zero (and happen to have a band structure of 5 diagonals). If I were writing this matrix on a blackboard, I would put a big zero in the middle of the zero blocks, perhaps separate the blocks out with a horizontal and vertical line and also put big zeros in the 4 triangles that form around the band structure. I would also have diagonal showing the band structure.

My knowledge of LaTeX allows me to place \vdots, \hdots and \ddots inside an array but this is less than satisfying. Is there a nice solution for making beautiful band structured matrices?

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I think you want to look into the easybmat package.
$\left( \begin{BMAT}(rc){c:c}{c:c} 0 & \begin{BMAT}(rc){ccc}{ccc} 1 & &\\ & 2 &\\ & & 3 \end{BMAT}\\ \begin{BMAT}(rc){ccc}{ccc} 4 & &\\ & 5 &\\ & & 6 \end{BMAT} & 0 \end{BMAT} \right)$