Regrettable, just as you have it.
The root word of "regrettable" is "regret."
That is the proper spelling, "thought" for the process of thinking.
That is the correct spelling of the word "proper" (correct, appropriate).The proper spelling is "proper" (correct, appropriate).
The proper spelling for "Granddad" is with two d's.
The US spelling is center but the spelling "centre" is frequently used for proper nouns such as buildings and towns.The UK spelling is centre.
The proper spelling is low fat.
The correct spelling is addition.
The proper spelling of misplaced is "misplaced". It means to put something in a wrong position.
The tragedies of the past have left a deep and profoundly regrettable legacy of suffering.
The proper spelling is "difference" (no A's).
Helps to teach you proper spelling.
That is the correct spelling, although 'hifalutin' is also an acceptable spelling.