A simple option is to check for matches before you replace:

var regex = /i/g;

var newStr = str;



var replaced = str.search(regex) >= 0;

if(replaced){

    newStr = newStr.replace(regex, '!');

}

If you don't want that either, you can abuse the replace callback to achieve that in a single pass:

var replaced = false;

var newStr = str.replace(/i/g, function(token){replaced = true; return '!';});