I'm working on making a platformer in Flash and so far have an okay script for making my character speed up to a maximum walking speed quickly, so the movement isn't jerky. The only problem I have is once the variable of the walking speed gets all the way up, it stays this way. If I press the left or right key again, the movement speed is still at 5 so it's jerky and I really want to find a way to have smooth acceleration when moving in my game. I'm trying to implement a way to check if the key is down, so I can set the speed back to 0 if a key is released, but I'm not really sure how. This is my code so far. I'm not sure if I need variables for my left and right speed, but I thought it might come in handy in the future.

Code:
onClipEvent (load)
{
	rightSpeed=0;
	leftSpeed=0;
}

onClipEvent (enterFrame)
{
	if (Key.isDown(Key.RIGHT)) 
	{
		rightSpeed++;
		if (rightSpeed > 5)
		{
			rightSpeed=5;
		}
		this._x+=rightSpeed;
	}

	if (Key.isDown(Key.LEFT)) 
	{
		leftSpeed++;
		if (leftSpeed > 5)
		{
			leftSpeed=5;
		}
		this._x-=leftSpeed;
	}	
}
To rephrase my question, how could I do an event like Key.isUp? I know this doesn't exist, except with key listeners but I can't figure out how to use a listener. If someone could help me that would be great.

Thanks
Ethan