Quick question, is it possible to lose being a regular by being inactive?
yes, i lost mine due to being inactive. i think you have to be active a certain number of time in the last hundred days to maintain the status. it is possible to earn it back though
if i can find ribblr’s article about it i will link it
“Please note: unlike other community levels, you can lose a Regular level if you dip below these requirements in the last 100 days, and you will be switched back to a Member level. However, there is a 4-week grace period immediately after gaining a Regular level.“
this is what the community wiki says about losing the status 
and here is the full wiki if you want to read more about how to become a regular 