How do you regularly review programming syntax without forgetting it?

Try Anki for spaced repetition. Make short cards from your own notes, especially for syntax you keep mixing up. A prompt could ask you to write a small example rather than just recognize the answer.

Review the cards regularly and let the app schedule them. You can also set a calendar reminder if you want a separate nudge to open it.

Write little programs instead of trying to memorize a whole language. A calculator is enough to give you a reason to use variables, conditions, and functions. Change it, break it, and figure out how to fix it. The syntax you use often will get more familiar through repetition.

Don't worry about recalling every detail from memory. Keep your original study materials handy and look things up when you need them. Put more effort into understanding what data types and structures do, so you know what to look for when the syntax slips your mind.