[R] regular expression question

[markleeds at verizon.net]

I have strings of the form

TICKER.GGG.XXXXXX.dat

but GGG is not always three characters so I can't use substr to pull it 
out of the string.

Could someone tell me how to use sub to pull out the GGG but more 
generally the string between the dot after the R in TICKER and the next
dot.  I still don't have a very good understanding of regular 
expressions but I'm trying to get that slowly. thanks.