You can use functions unlist() and gregexpr() to find the position of a character in a string. If the character appears more than once in the string, you will get all positions.
Here is an example:
> r = "helloarielprinter"
> unlist(gregexpr('p', r))
[1] 11
> unlist(gregexpr('e', r))
[1] 2 9 16
The first argument in the function gregexpr() is the character you are trying to search for, and the second argument is the string.