せざるを得ない — is the する conjugation really irregular here?

Question

Working through ざるを得ない and got tripped up on する verbs. My gut said しざるを得ない (just drop ない from しない and add ざる), but that looks wrong every time I see it in the wild. Is there a special exception for する I'm missing?

Accepted Answer

Yes, する is irregular here — it becomes せざるを得ない, not しざるを得ない. So the pattern is: drop ない from the plain negative, replace with ざる, then add を得ない. For する specifically you use the せ- stem instead of し-. I see this constantly in work emails, e.g. 残業せざるを得ない ("I have no choice but to work overtime").

Answer

To add the historical note since it helps some people remember it: ざる is the classical negative auxiliary (from ず), and する's classical negative stem is せ, not し — that's why it's frozen as せざる instead of しざる. Same reason you see せねば in old-fashioned obligation phrases.

Answer

This tripped me up too when I first read a Japanese news article — 説明せざるを得ない ("they have no choice but to explain") shows up a lot in reporting. Just memorize せざるを得ない as one chunk and it stops feeling weird.