Sudoku (Fill-in)

Sudoku is a number puzzle where a grid of 81 digits (9×9) is filled by the digits 1-9 such that no row, column, or 3×3 subregion contains duplicate digits.

Write a program that given an incomplete Sudoku board as an argument, prints the solved Sudoku board. The grid will be drawn with Unicode box-drawing characters like so:

┏━━━┯━━━┯━━━┳━━━┯━━━┯━━━┳━━━┯━━━┯━━━┓
┃ 2 │ 5 │ 8 ┃ 4 │ 1 │ 7 ┃ 6 │ 9 │ 3 ┃
┠───┼───┼───╂───┼───┼───╂───┼───┼───┨
┃ 6 │ 1 │ 7 ┃ 9 │ 2 │ 3 ┃ 8 │ 5 │ 4 ┃
┠───┼───┼───╂───┼───┼───╂───┼───┼───┨
┃ 9 │ 3 │ 4 ┃ 8 │ 6 │ 5 ┃ 1 │ 7 │ 2 ┃
┣━━━┿━━━┿━━━╋━━━┿━━━┿━━━╋━━━┿━━━┿━━━┫
┃ 3 │ 2 │ 5 ┃ 7 │ 8 │ 1 ┃ 4 │ 6 │ 9 ┃
┠───┼───┼───╂───┼───┼───╂───┼───┼───┨
┃ 8 │ 9 │ 6 ┃ 3 │ 5 │ 4 ┃ 2 │ 1 │ 7 ┃
┠───┼───┼───╂───┼───┼───╂───┼───┼───┨
┃ 7 │ 4 │ 1 ┃ 6 │ 9 │ 2 ┃ 5 │ 3 │ 8 ┃
┣━━━┿━━━┿━━━╋━━━┿━━━┿━━━╋━━━┿━━━┿━━━┫
┃ 4 │ 6 │ 9 ┃ 1 │ 3 │ 8 ┃ 7 │ 2 │ 5 ┃
┠───┼───┼───╂───┼───┼───╂───┼───┼───┨
┃ 5 │ 7 │ 3 ┃ 2 │ 4 │ 6 ┃ 9 │ 8 │ 1 ┃
┠───┼───┼───╂───┼───┼───╂───┼───┼───┨
┃ 1 │ 8 │ 2 ┃ 5 │ 7 │ 9 ┃ 3 │ 4 │ 6 ┃
┗━━━┷━━━┷━━━┻━━━┷━━━┷━━━┻━━━┷━━━┷━━━┛

Variants: Sudoku

External links: Rosetta Code, Wikipedia