Alternately, if it uses a regular expression or other form of pattern matching, it's very likely that the original coder made unit tests for the most common beh

Alternately, if it uses a regular expression or other form of pattern matching, it's very likely that the original coder made unit tests for the most common behaviors, but built in assumptions for edge cases. And I'm unaware of any code coverage tools that help someone test every single branch in a regex. So you're reliant on comments, documentation, etc. (thank goodness for Python's verbose regex mode, for example) to make those assumptions explicit. It's not an easy problem to solve.