> As of about 2 years ago, expr and relatives are LGPL and thus compatible with the App Store.

I emailed the Free Software Foundation, who are the publishers of the LGPL. They responded

that the LGPL is not compatible with the restrictive terms of Apple's app store.

I forgot to ask them in advance if I could publicly post their response. If they say I can do so, I'll post their actual response. But it sounded pretty clear.

I suppose one could argue if it's a free app then who cares, and that the copyright holders of expr (or even Apple) are extremely unlikely to create a licensing fuss. But then that's the case whether expr is GPL, LGPL, or even "don't use this to murder people with drones" license.

If you want to write/use open source software that's compatible with Apple's app store, use

the 3-clause BSD license.

-Jonathan