using greeter as base for locking app

I am a hobbyist (not IT specialist) and just had idea to write some lockscreen app/script to learn and repeat regex. I could use bash or Python, but don't know where to start. Could you point me directions? We are using Operating Systems based on X (Manjaro and Ubuntu) And please do not hate me as a noob. I always wanted to have knowledge in programming area, but never been determined enough to spend time on this (my wife hates computers at all). But some time ago I sat down (and started to learn) and wrote Python app to determine time my son spends playing on computer. It took me 3 months but finally it worked as expected - I designed it myself, I wrote including refactoring and the conversion to an exe file, I went through the update because my son broke the first security measures, and I think it's great fun :) So as mentioned earlier I need some tool (wrote by myself of course) to repeat regex, and I think locking screen will be best. Maybe I could use greeter (In first line it should print some text instead of user, and unlocking password will be matching regex) It is just idea, which I wanted to install on my sons computer too (hes 15 now and learns for IT specialist). Greeter is only my suggestion. Maybe there is something better to use. Could you please guide me ? Thanks in advance :)