R base has many functions for regular expressions, see slide 9 of Ed's talk below. The package stringr, created by Hadley Wickham, is a nice alternative that wraps the base regex functions for easier use. I highly recommend stringr.
Ed Goodwin, the coordinator of the Houston R Users group, gave a presentation to the group last night on regular expressions in R. It was a great talk, and he is allowing me to post his talk here.
Enjoy! And thanks for sharing Ed!