No there is not room under any of the seats on an ERJ for a small backpack, much less a full backpack. Even the overhead bins will only handle small items. You will have to gate check the backpack at the gate, usually leaving it on the jetbridge before you board, and pick it up at the destination immediately after deplaning.
The RJs can only handle very small carryon items, like a laptop bag, purse, etc.
Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.