The phrase "a regrettable development" is correct and usable in written English.
It can be used to describe a situation or event that is unfortunate or undesirable.
Example: "The recent increase in pollution levels is a regrettable development that we must address immediately."
Alternatives: "an unfortunate occurrence" or "a disappointing situation".
In another regrettable development, I am no longer thirteen, and apparently won't be again.
The Egyptian interior ministry issued a statement saying it "fully appreciated" the emotions surrounding "regrettable developments" in the region, but would not permit unauthorised protests.
Jordan's King Abdullah II appealed to his countrymen late Friday to express their anguish for the Iraqi people in a "civilized way". The Egyptian Interior Ministry, in an unusually conciliatory move, said it appreciated the emotions surrounding "regrettable developments" in the region and said it would allow demonstrations -- if the protesters had official permission.
I believe no one of insight from the US side would like to see that the election factors would have a regrettable impact on the development of ties between the two countries … We should deal with friction and differences in bilateral economic and trade cooperation in the spirit of seeking mutual benefits and win-win results through a positive and constructive way".
Along this line, it is a regrettable fact that software development is often discounted in the scientific community.
Various authorities have directed their efforts towards reducing the burden of malaria which has catastrophic effects on household socio-economic development and leads to a regrettable retardation of national economic growth.
There is, however, a regrettable shortage of both, intrinsic to the development of thoroughbred race horses.
The truth is as murky as George's photographs, which have a regrettable tendency of fading to black soon after development.
That's a regrettable situation".
