Irregular Sudoku (Mini Sudoku Series #19)

Mini Irregular Sudoku for the practice of Sudoku Champs 2015 Irregular Sudoku a many times very difficult to understand for newcomers. It is recommended for newcomers to try 6x6 Irregular Sudoku first to get familiarize with this Sudoku type before solving 9x9 Irregular Sudoku.
Irregular Sudoku is also known as Jigsaw Sudoku. I am posting this Irregular Sudoku as the 19th Sudoku in Mini Sudoku Series. Can you solve this Sudoku puzzle?

Rules of Irregular Sudoku

Place a digit from 1 to 6 in each empty cell so that each digit appears exactly once in each row, column, and outlined region. Each outlined region is marked by thick borders.