r gsub - replace only first match of a word [duplicate]

Question

I have a string like this: "21st-August-2017" and I would like to replace the "st" after 21 with "xx" so that the result is

"25xx-August-2017"

I have tried using regex repetition quantifiers, e.g.

gsub("st{1}", "xx", "21st-August-2017")

But this still replaces both instances of "st". How can I specify that it should match only the first instance of "st"?

Answer 1

Please use sub instead of gsub.

Replace the first occurrence of a pattern with sub or replace all occurrences with gsub.