[GRASS-user] to define a region which is not rectangular

[FAROUX STEPHANIE]

Hi,
In my raster map sea pixels and missing data have the same null value. 
I'd like to separate the two but i don't have a sea/land mask. Missing 
data are concentrated in some areas.
How can i do it?
I thought to define regions in which missing data would be gathered but 
if i only use the zoom to do it, it will take a long time.
Maybe using vectors?
Can anybody help me?
Thank you.
Stéphanie