First things first, the reach-in foul is not formally defined by any basketball rules (including NBA rules). Fans and journalists commonly use the terms “reach-in” or “reaching in” when defenders come in illegal contact with an opponent in possession of the ball. girls basketball
The so-called reach-in foul occurs when a defending player reaches toward the orange in an attempt to steal the ball but instead illegally contacts the ball handler.
The act of reaching in by itself is not considered a foul in any rulebook, be it NBA or FIBA rulebooks. However, depending on the circumstances, it may indeed be a foul.
We’ll have to take a look at the NBA rulebook to understand when it is and when it is not an offense.