removing characters from a string

gsub("[a-zA-Z]", "", "aB9c81")

From: Vivek Rao

Is there a simple way in R to remove all characters
from a string other than those in a specified set? For
example, I want to keep only the digits 0-9 in a
string.

In general, I have found the string handling abilities
of R a bit limited. (Of course it's great for stats in
general). Is there a good reference on this? Or should
R programmers dump their output to a text file and use
something like Perl or Python for sophisticated text
processing?

I am familiar with the basic functions such as nchar,
substring, as.integer, print, cat, sprintf etc.