I have a dataframe in R with a column of strings, e.g. `v1 <- c('JaStADmmnIsynDK', 'laUksnDTusainS')`

My goal is to capitalize all letters in each string except 's', 't' and 'y'. 

So the result should end up being: 'JAStADMMNIsyNDK' and 'LAUKsNDTUsAINS'.

Thus not changing any of the said letters: 's', 't' and 'y'.

As of now I do it by simply having 25x

```
levels(df$strings) <- sub('n', 'N', levels(df$strings))

```

But that seems to be overkill! How can I do this easily in R?