Read the blog entry for this solver here.
There are so many ways to do this, and I've tried a lot of them in PHP, Perl, C, and Python. My Sudoku blog post describes a few of my attempts, and may one day have github.com links to the scripts.
2023-01-07 Boston Globe (Friday) 2023-01-13 Boston Globe (Friday) 2023-01-15 Boston Globe (Sunday) 2023-03-12 Boston Globe (Sunday, four tries!) Boston Globe 2012-06-10 Boston Globe 2012-06-11 Boston Globe 2012-06-12 Boston Globe 2012-06-13 Boston Globe 2012-06-14 Boston Globe 2012-06-15 Boston Globe 2012-06-16 Boston Globe 2012-06-17 Boston Globe 2012-06-18 Boston Globe 2012-06-19 Boston Globe 2015-11-07 Boston Globe 2022-12-03 Boston Globe 2022-12-04 Boston Globe Sunday Boston Globe, 2023-02-22 Boston Globe, 2023-02-25 Boston Sunday Globe 2022-12-18 Very difficult