How can I replace a match only at a certain position inside the string?

Question

So, I have a function which has two params: string and match index to replace and i need to replace only match with that index. How can i do that?

Example:

Answer 1

For those like me who find regex to be cryptic, here is "pure JavaScript" way as well:

function CustomReplace(strData, strTextToReplace, strReplaceWith, replaceAt) {
    var index = strData.indexOf(strTextToReplace);
    for (var i = 1; i < replaceAt; i++)
        index = strData.indexOf(strTextToReplace, index + 1);
    if (index >= 0)
        return strData.substr(0, index) + strReplaceWith + strData.substr(index + strTextToReplace.length, strData.length);
    return strData;
}

Usage:

var mystr = 'a_a_a_a_a';
var newstr = CustomReplace(mystr, "_", "__", 2); //replace the second appearance

Live test case: http://jsfiddle.net/tXx5n/2/