[R] ?replace characters within vector data

[Alexander.Herr at csiro.au]

Hi List,

I want  replace characters within a vector. Outside R I could use sed,
but I'd like to automate it in R. For example

vectorx
xxxyyz
xxxyyza
xxxyyzzb

I want to change to: 

vectorx
aaayyz
aaayyza
aaayyzzb

The obvious replace command only deals with whole data entries?
Any hints would be appreciated.

Thanks
Herry