Replace Nth Match (from http://stackoverflow.com/a/7958627/2794906)

replaceNthMatch.js

var replaceNthMatch = function (original, pattern, n, replace) {
	var parts, tempParts;

	if (pattern.constructor === RegExp) {

		// If there's no match, bail
		if (original.search(pattern) === -1) {
			return original;
		}

		// Every other item should be a matched capture group;
		// between will be non-matching portions of the substring
		parts = original.split(pattern);

		// If there was a capture group, index 1 will be
		// an item that matches the RegExp
		if (parts[1].search(pattern) !== 0) {
			throw {
				name: "ArgumentError",
				message: "RegExp must have a capture group"
			};
		}
	} else if (pattern.constructor === String) {
		parts = original.split(pattern);
		// Need every other item to be the matched string
		tempParts = [];

		for (var i = 0; i < parts.length; i++) {
			tempParts.push(parts[i]);

			// Insert between, but don't tack one onto the end
			if (i < parts.length - 1) {
				tempParts.push(pattern);
			}
		}
		parts = tempParts;
	} else {
		throw {
			name: "ArgumentError",
			message: "Must provide either a RegExp or String"
		};
	}

	// Parens are unnecessary, but explicit. :)
	indexOfNthMatch = (n * 2) - 1;

	if (parts[indexOfNthMatch] === undefined) {
		// There IS no Nth match
		return original;
	}

	if (typeof (replace) === "function") {
		// Call it. After this, we don't need it anymore.
		replace = replace(parts[indexOfNthMatch]);
	}

	// Update our parts array with the new value
	parts[indexOfNthMatch] = replace;

	// Put it back together and return
	return parts.join('');

}