Replacing the nth instance of a regex match in Javascript

Question

I\'m trying to write a regex function that will identify and replace a single instance of a match within a string without affecting the other instances. For example, I have

Answer 1

here's something that works:

"23||45||45||56||67".replace(/^((?:[0-9]+\|\|){n})([0-9]+)\|\|/,"$1$2&&")

where n is the one less than the nth pipe, (of course you don't need that first subexpression if n = 0)

And if you'd like a function to do this:

function pipe_replace(str,n) {
   var RE = new RegExp("^((?:[0-9]+\\|\\|){" + (n-1) + "})([0-9]+)\|\|");
   return str.replace(RE,"$1$2&&");
}