The only dictionary currently listing
regen is Wiktionary, as far as I can see. But it makes up for the others by giving several meanings, including an informal term for
regeneration and to
regenerate, as well as regenerative braking.
Looking at usage patterns, it seems the word has been used quite often in the last few years, mainly for regenerative braking. A recent example is from
Autocar magazine in New Zealand a few days ago:
When you’re driving around in Eco, the paddles act as brake regen settings of which there are three, along with a coast mode.
The word is occasionally used as a verb. For example on the
Driving website from Canada in October 2021:
The system is smart, as it looks at navigation data and other sensors to determine what’s happening — close in on a slower-moving vehicle and the system automatically begins to regen to reduce speed.
I'll add
regen,
regenning and
regenned as rare words.