regex for preserving case pattern, capitalization

This is one of those occasions when I think a for loop is justified:

input <- rep("Here are a date, a Date, and a DATE",2)
pat <- c("date", "Date", "DATE")
ret <- c("month", "Month", "MONTH")

for(i in seq_along(pat)) { input <- gsub(pat[i],ret[i],input) }
input
#[1] "Here are a month, a Month, and a MONTH" 
#[2] "Here are a month, a Month, and a MONTH"

And an alternative courtesy of @flodel implementing the same logic as the loop through Reduce:

Reduce(function(str, args) gsub(args[1], args[2], str), 
       Map(c, pat, ret), init = input)

For some benchmarking of these options, see @TylerRinker's answer.

Using the gsubfn package, you could avoid using nested sub functions and do this in one call.

> library(gsubfn)
> x <- 'Here we have a date, a different Date, and a DATE'
> gsubfn('date', list('date'='month','Date'='Month','DATE'='MONTH'), x, ignore.case=T)
# [1] "Here we have a month, a different Month, and a MONTH"