Plan for touching legacy code

You need to change something nobody understands and there are no tests.

I need to change this code. It is old, has no tests, and I am not confident about what depends on it.

Help me do it safely:
1. What does this code appear to guarantee to its callers? What might they rely on that isn't obvious?
2. What characterisation tests should I write *first*, to lock in current behaviour before I change anything?
3. What is the smallest change that achieves my goal?
4. What is the order of operations that keeps it working at every step?
5. What should I watch after deploying to know whether I broke something?

Code:


What I need to change:

Understanding code you inherited. Making sense of an unfamiliar file, line or regex before you dare change it.