"Reaching in" Against a Dribbler
"Reaching in" itself is not a foul. However, if a defender reaches in to steal possession from the offensive player, the defender must not make contact that, according to the NBA rulebook, "results in a re-routing of an opponent." The rulebook also says that "incidental contact with the hand against an offensive player shall be ignored if it does not affect the player's speed, quickness, balance and/or rhythm."
An exception to "reaching in" would be contacting the offensive player's hand while it is touching the ball. When a player's hand is in contact with the ball, the hand is considered part of the ball.
Reaching "Over the Back"
Reaching over an offensive player's back to block a shot is legal as long as the defender does not make illegal contact with the offensive player's body. The defensive player can legally contact the ball or a hand in contact with the ball when attempting to block a shot over the offensive player's back. Any other contact that affects the player's balance or rhythm is a foul.