Sudoku rules

Sudoku is played over a 9x9 grid, divided to 3x3 sub grids called "regions".
Sudoku begins with some of the grid cells already filled with numbers.

The object of Sudoku is to fill the other empty cells with numbers between 1 and 9 (1 number only in each cell) according to the following guidelines.

1. Number can appear only once on each row.
2. Number can appear only once on each column.
3. Number can appear only once on each region.

A summary of these guidelines would be, that a number should appear only once on each row, column and a region.