[R] Retrieve regular expression groups

[OKB (not okblacke)]

    	I'm trying to figure out how to get the text captured by capturing 
groups out of a regex match.  For instance, let's say I have the pattern 
"foo ([^ ]+)" and I match it against the string "This is a foo sentence 
I am reading."  The group in the pattern will match the word "sentence" 
in the target string.  How can I get access to this matched group?  All 
I can seem to get the various grep/gsub functions to do is return or 
modify the entire target string.  Isn't there a way to extract ONLY the 
text from a particular group or groups?

Thanks,
-- 
--OKB (not okblacke)
Brendan Barnwell
"Do not follow where the path may lead.  Go, instead, where there is
no path, and leave a trail."
	--author unknown