[R] Problem with the grep function

[aubert@inapg.fr]

Let me present to you my problem :

I have a character vector x and I would like to  obtain the indices of the 
elements of
this vector that yielded exactly a match.

For example,  x=nom, pattern="b", I would to obtain 2 because "b" is on the 
second position.

First program :
nom <- c("a","b","ab")
grep("b",nom)
2 3

Then I try the option extended =FALSE (instead of TRUE by default) and I 
obtain '2 3' a second time.

Please can you help me : How can I obtain only 2 in using the grep function 
(without using the match function).

Thanks you

Julie AUBERT