To date, my most complex piece of software in terms of how it was implemented was a simple calculator written in my first programming course. About a week before I started it, I had learned about regular expressions (the programming construct, not the formal language construct).

The basic implementation was a calculator that could add, sub, mul, div, pow, and sqrt. Bonus points were awarded for adding additional functionality including lettered variables. It started with a relatively clean shunting-yard implementation, but my use of regular expressions quickly fixed that.

My calculator worked under most circumstances, but that thing is an eye sore. I'll never get rid of that source code. I like to go back and look a few key pieces of software I've written through the years.