If I write code for a personal project, can I use it in future jobs?

The problem you have is tricky but do-able.

If you have personally developed code that you want to use in a paid job you will want to identify that code in the program with comments and also send some communications to your boss that you are using some code segments, modules, routines, libraries (or whatever they are) that you had personally developed and retain ownership of.

Keep any e-mail communications on this topic in case any questions or disputes arise later on.

Best Wishes!

This will be your personal intellectual property. If it is especially unique, see about getting exclusive ownership of it by visiting an attorney. Otherwise, your next employer will claim it as theirs if you allow them to use it while employed there. Then you will not be able to port it to the next job.