[R] How to transform the date format as "20050425"

[Yong Wang]

Dear R user,
if the dates are in format as "20050425" i.e., Apr. 25 2004"
can you suggest an easy way to transfom it to standard form
as "2005-04-25" or "2004Apr25" or "2005/04/25" or any other
format which is R recognizable?
if there is no easy way to do that, can you let me know what
is the function in R performing similiar function as the "string"
function in C or some other more basic language, so I can loop
through all dates to make the desired change.
thank you
regards