Update index with a pickaxed diff

I'd like to know if there's a quick way of updating the index with the output of 'git diff -S<expr>'. I find this useful if the change set that I wanted to include results from, for example, a mass identifier renaming. That would be easier, especially when my working directory is already dirty with other changes, and I don't have to interactively filter out change sets, other than those matching <expr>.

Thanks.