regex for preserving case pattern, capitalization

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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.

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