Animated masking a line

Ok, here is what I have,
I have a basic square and a wavy line graphic on the stage. Now all I want is to animate a mask unveiling the line. Kinda like the line draws itself. I’m just not sure of how to go about it. I would love to do this with actionscript, but not sure about that either.
Anyhelp would be greatly appreciated.
THanks
(Texas jr. Flasher)